Support cross-origin frontends alongside same-origin setups

- django-cors-headers with env-driven CORS_ALLOWED_ORIGINS,
  CORS_ALLOW_ALL_ORIGINS, CORS_ALLOW_CREDENTIALS and CSRF_TRUSTED_ORIGINS;
  same-origin traffic is unaffected and a disallowed origin gets no CORS
  headers. Token auth needs no cookies, so credentials stay off by default.
- TRUST_PROXY_HEADERS=true lets a TLS-terminating proxy supply
  X-Forwarded-Proto/Host for correct absolute URLs.
- API media URLs (raw/thumbnail/upload/similarity/staged previews) are now
  absolute, built from the request host, so <img>/<video>/fetch() keep
  working when the SPA is served from another origin. Signed URLs are still
  per-user; nothing is stored in the DB.
- The SPA gains VITE_API_BASE (build-time, empty = same-origin) applied by
  a small apiUrl() helper used for XHR/fetch and the few URL fallbacks.

Verified with a throwaway instance: preflight and GET responses carry the
allowed origin, foreign origins get nothing, media GETs include CORS for
cross-origin fetch(), and payload URLs use the request host (dev :8000
unchanged).

frontend/src/features/library/DuplicatesPage.tsx

@@ -3,7 +3,7 @@ import { useMemo, useState } from "react";
 import { Link } from "react-router-dom";
 
 import { Button, Spinner, inputClass } from "@/components/ui";
-import { api, errorMessage } from "@/lib/api";
+import { api, apiUrl, errorMessage } from "@/lib/api";
 import { cn } from "@/lib/cn";
 import { formatBytes } from "@/lib/format";
 import type {
@@ -35,8 +35,9 @@ function MemberCard({
   selected: boolean;
   onToggle: () => void;
 }) {
-  const preview =
-    entry.thumbnail_url ?? `/api/files/${entry.j_id}/thumbnail/`;
+  const preview = apiUrl(
+    entry.thumbnail_url ?? `/api/files/${entry.j_id}/thumbnail/`,
+  );
   return (
     <div
       className={cn(
@@ -250,10 +251,10 @@ export default function DuplicatesPage() {
                 <div className="w-32 shrink-0">
                   <div className="aspect-square overflow-hidden rounded-md bg-ctp-mantle">
                     <img
-                      src={
+                      src={apiUrl(
                         group.thumbnail_url ??
-                        `/api/files/${group.j_id}/thumbnail/`
-                      }
+                          `/api/files/${group.j_id}/thumbnail/`,
+                      )}
                       alt={group.j_id}
                       loading="lazy"
                       className="h-full w-full object-cover"