IQDB reverse search from the library detail

- New IQDB card on local item pages: fetches the signed raw file, POSTs it
  to e621's /iqdb_queries.json with the user's credentials, and lists
  candidates as tiles (thumbnail, rating, score%) with exact-MD5 markers.
- Selecting a candidate opens a detail panel (rating, score, favs, size,
  tag preview) and linking is an explicit confirm that reuses the e621
  match endpoint; videos show a note since IQDB is image-only.
- Guests don't see the card; linking follows the uploader/staff rule.

ROADMAP.md

@@ -39,9 +39,11 @@ they land.
         `manage.py match_e621`
   - [x] Show match status + e621 metadata in the library: status badge and
         match card on the detail, and `not_found` / `deleted` status filters
-- [ ] **IQDB reverse search**
-  - [ ] Search from a local file (`POST /iqdb_queries.json`)
-  - [ ] Show candidate posts to link
+- [x] **IQDB reverse search**
+  - [x] Search from a local file (`POST /iqdb_queries.json` from the library detail,
+        using the signed raw file)
+  - [x] Show candidate posts to link (thumbnail, rating, score, favs, tags;
+        select then confirm — same flow as uploads, exact MD5 marked)
 - [ ] **Follows**
   - [ ] Follow tags/pools, Followed screens with unseen badges
   - [ ] "Followed" nav entry

frontend/src/features/library/IqdbCard.tsx

@@ -0,0 +1,282 @@
+import { useMutation, useQueryClient } from "@tanstack/react-query";
+import { ScanSearch } from "lucide-react";
+import { useState } from "react";
+import { Link } from "react-router-dom";
+
+import { Button, Spinner } from "@/components/ui";
+import { api, errorMessage } from "@/lib/api";
+import { cn } from "@/lib/cn";
+import {
+  e621ErrorMessage,
+  effectiveCredentials,
+  iqdbSearch,
+  RATING_LABELS,
+  type E621IqdbResult,
+} from "@/lib/e621";
+import type { MediaItem } from "@/lib/types";
+import { useE621 } from "@/store/e621";
+
+const ratingPill: Record<string, string> = {
+  s: "bg-ctp-green text-ctp-crust",
+  q: "bg-ctp-peach text-ctp-crust",
+  e: "bg-ctp-red text-ctp-crust",
+};
+
+type Notice = { kind: "ok" | "warn" | "error"; text: string } | null;
+
+export function IqdbCard({
+  item,
+  canLink,
+}: {
+  item: MediaItem;
+  canLink: boolean;
+}) {
+  const queryClient = useQueryClient();
+  const credentials = useE621((state) => state.credentials);
+  const [busy, setBusy] = useState(false);
+  const [candidates, setCandidates] = useState<E621IqdbResult[] | null>(null);
+  const [selected, setSelected] = useState<E621IqdbResult | null>(null);
+  const [notice, setNotice] = useState<Notice>(null);
+
+  const isImage = item.kind === "image";
+  const configured = Boolean(credentials?.configured);
+
+  async function search() {
+    setBusy(true);
+    setNotice(null);
+    setSelected(null);
+    setCandidates(null);
+    try {
+      const response = await fetch(item.raw_url);
+      if (!response.ok) {
+        throw new Error(`Could not load the file (HTTP ${response.status}).`);
+      }
+      const blob = await response.blob();
+      const results = await iqdbSearch(
+        effectiveCredentials(credentials),
+        blob,
+        item.filename,
+      );
+      setCandidates(results);
+      if (results.length === 0) {
+        setNotice({ kind: "warn", text: "No similar posts found on e621." });
+      }
+    } catch (error) {
+      setNotice({ kind: "error", text: e621ErrorMessage(error) });
+    } finally {
+      setBusy(false);
+    }
+  }
+
+  const linkMutation = useMutation({
+    mutationFn: (postId: number) =>
+      api<MediaItem & { md5_match?: boolean }>(
+        `/api/files/${item.j_id}/match/`,
+        { method: "POST", json: { post_id: postId } },
+      ),
+    onSuccess: (data) => {
+      void queryClient.invalidateQueries({ queryKey: ["file", item.j_id] });
+      void queryClient.invalidateQueries({ queryKey: ["files"] });
+      void queryClient.invalidateQueries({ queryKey: ["tag-cloud"] });
+      setSelected(null);
+      setNotice(
+        data.md5_match
+          ? {
+              kind: "ok",
+              text: `Linked to post #${data.e621_post_id} — MD5 matches.`,
+            }
+          : {
+              kind: "warn",
+              text: `Linked to post #${data.e621_post_id}, but its MD5 differs from this file.`,
+            },
+      );
+    },
+    onError: (error) =>
+      setNotice({ kind: "error", text: errorMessage(error) }),
+  });
+
+  return (
+    <section className="rounded-lg border border-ctp-surface0 bg-ctp-base p-4">
+      <div className="flex items-center justify-between gap-2">
+        <h2 className="text-sm font-semibold text-ctp-subtext1">IQDB search</h2>
+        {candidates && candidates.length > 0 ? (
+          <span className="font-mono text-[11px] text-ctp-overlay0">
+            {candidates.length} candidates
+          </span>
+        ) : null}
+      </div>
+
+      <p className="mt-2 text-xs text-ctp-overlay0">
+        Reverse-search e621 with this file to find its original post.
+      </p>
+
+      <Button
+        variant="secondary"
+        className="mt-3 w-full"
+        disabled={busy || !isImage || !configured}
+        onClick={() => void search()}
+      >
+        {busy ? (
+          <Spinner className="h-3.5 w-3.5" />
+        ) : (
+          <ScanSearch className="h-4 w-4" />
+        )}
+        {busy ? "Searching…" : "Search e621"}
+      </Button>
+
+      {!configured ? (
+        <p className="mt-2 text-[11px] text-ctp-overlay0">
+          Configure e621 credentials in Account to use IQDB.
+        </p>
+      ) : !isImage ? (
+        <p className="mt-2 text-[11px] text-ctp-overlay0">
+          IQDB works on images only.
+        </p>
+      ) : null}
+
+      {notice ? (
+        <p
+          className={cn(
+            "mt-2 text-xs",
+            notice.kind === "ok"
+              ? "text-ctp-green"
+              : notice.kind === "warn"
+                ? "text-ctp-yellow"
+                : "text-ctp-red",
+          )}
+        >
+          {notice.text}
+        </p>
+      ) : null}
+
+      {candidates && candidates.length > 0 ? (
+        <div className="mt-3 grid grid-cols-3 gap-1.5">
+          {candidates.map((candidate) => {
+            const exact = candidate.md5 !== null && candidate.md5 === item.md5;
+            return (
+              <button
+                key={`${candidate.post_id}-${candidate.md5}`}
+                type="button"
+                disabled={linkMutation.isPending || candidate.post_id === null}
+                onClick={() => setSelected(candidate)}
+                className={cn(
+                  "overflow-hidden rounded-md border bg-ctp-mantle text-left transition",
+                  selected?.post_id === candidate.post_id
+                    ? "border-ctp-mauve ring-2 ring-ctp-mauve/40"
+                    : "border-ctp-surface0 hover:border-ctp-surface2",
+                )}
+              >
+                <span className="relative block aspect-square overflow-hidden">
+                  {candidate.preview_url ? (
+                    <img
+                      src={candidate.preview_url}
+                      alt={`Post #${candidate.post_id}`}
+                      loading="lazy"
+                      className="h-full w-full object-cover"
+                    />
+                  ) : (
+                    <span className="flex h-full w-full items-center justify-center text-[10px] text-ctp-overlay0">
+                      No preview
+                    </span>
+                  )}
+                  {candidate.rating ? (
+                    <span
+                      className={cn(
+                        "absolute left-1 top-1 rounded-full px-1 py-0.5 font-mono text-[9px] font-semibold uppercase",
+                        ratingPill[candidate.rating] ??
+                          "bg-ctp-surface1 text-ctp-text",
+                      )}
+                    >
+                      {candidate.rating}
+                    </span>
+                  ) : null}
+                  {exact ? (
+                    <span className="absolute right-1 top-1 rounded-full bg-ctp-green/90 px-1 py-0.5 font-mono text-[9px] font-semibold uppercase text-ctp-crust">
+                      MD5
+                    </span>
+                  ) : null}
+                </span>
+                <span className="block px-1 py-0.5 font-mono text-[10px] text-ctp-overlay0">
+                  #{candidate.post_id}
+                  {candidate.score !== null
+                    ? ` · ${Math.round(candidate.score)}%`
+                    : ""}
+                </span>
+              </button>
+            );
+          })}
+        </div>
+      ) : null}
+
+      {selected ? (
+        <div className="mt-3 rounded-md border border-ctp-surface0 bg-ctp-surface0/40 p-3">
+          <div className="flex items-center justify-between gap-2">
+            <p className="font-mono text-xs text-ctp-subtext1">
+              Post #{selected.post_id}
+            </p>
+            {selected.md5 === item.md5 ? (
+              <span className="rounded-full bg-ctp-green/15 px-1.5 py-0.5 font-mono text-[9px] font-semibold uppercase text-ctp-green">
+                MD5 match
+              </span>
+            ) : null}
+          </div>
+          <p className="mt-1 text-xs text-ctp-overlay0">
+            {selected.rating
+              ? (RATING_LABELS[selected.rating] ?? selected.rating)
+              : "Unknown rating"}
+            {selected.score_total !== null
+              ? ` · ▲ ${selected.score_total}`
+              : ""}
+            {selected.fav_count !== null ? ` · ${selected.fav_count} favs` : ""}
+            {selected.width && selected.height
+              ? ` · ${selected.width}×${selected.height}`
+              : ""}
+          </p>
+          {selected.tags_preview.length > 0 ? (
+            <div className="mt-2 flex flex-wrap gap-1">
+              {selected.tags_preview.map((tag) => (
+                <span
+                  key={tag}
+                  className="rounded border border-ctp-surface1 px-1 py-0.5 font-mono text-[10px] text-ctp-subtext0"
+                >
+                  {tag}
+                </span>
+              ))}
+            </div>
+          ) : null}
+          <div className="mt-3 flex items-center gap-3">
+            <Button
+              disabled={
+                !canLink || linkMutation.isPending || selected.post_id === null
+              }
+              onClick={() =>
+                selected.post_id !== null &&
+                linkMutation.mutate(selected.post_id)
+              }
+            >
+              {linkMutation.isPending ? "Linking…" : "Link selected post"}
+            </Button>
+            <Link
+              to={`/detail/${selected.post_id}`}
+              target="_blank"
+              className="text-xs text-ctp-blue hover:underline"
+            >
+              View post
+            </Link>
+          </div>
+          <p className="mt-2 text-[11px] text-ctp-overlay0">
+            {canLink
+              ? "Linking stores this post's metadata on the item."
+              : "Only the uploader or staff can link posts."}
+          </p>
+        </div>
+      ) : null}
+
+      {candidates && candidates.length > 0 && !selected ? (
+        <p className="mt-2 text-[11px] text-ctp-overlay0">
+          Select a candidate first, then confirm the link.
+        </p>
+      ) : null}
+    </section>
+  );
+}

frontend/src/features/library/LibraryDetail.tsx

@@ -6,6 +6,7 @@ import { Link, useNavigate } from "react-router-dom";
 import { Button, EmptyState, Spinner, inputClass, linkButtonClass } from "@/components/ui";
 import { TagChip } from "@/components/TagChip";
 import { DetailShell } from "@/features/detail/DetailShell";
+import { IqdbCard } from "@/features/library/IqdbCard";
 import { api, errorMessage } from "@/lib/api";
 import { cn } from "@/lib/cn";
 import { isTypingTarget } from "@/lib/dom";
@@ -697,6 +698,10 @@ export function LibraryDetail({ jId }: { jId: string }) {
 
           <MatchCard key={item.j_id} item={item} canEdit={canEdit} />
 
+          {user ? (
+            <IqdbCard key={item.j_id} item={item} canLink={canEdit} />
+          ) : null}
+
           <section className="rounded-lg border border-ctp-surface0 bg-ctp-base p-4">
             <h2 className="text-sm font-semibold text-ctp-subtext1">
               Locations ({item.locations.length})