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