feat: perceptual-hash duplicate detection + grouped picker view

The Duplicates section was useless: SHA-256-only detection only caught
byte-identical files, not the actual duplicates a real library
accumulates (re-encoded JPEGs, screenshots, resized exports), and the
view was a flat date-sorted list with no grouping or actions. This
replaces the whole flow.

Detection
- New phash + duplicate_group_id columns on Photo, added via an
  idempotent ALTER TABLE pass in init_db (the project has no Alembic).
- Thumbs worker computes a 64-bit pHash from the original-resolution
  decoded frame just before the destructive thumbnail loop. Falls back
  silently — phash is nice-to-have, not a blocker for thumbnails.
- backfill_phashes Celery task fills in phashes for photos that
  predated the column, reading the existing thumb_large rather than
  re-decoding the original.
- regroup_duplicates service runs union-find over Hamming distance
  (threshold 6), persists duplicate_group_id, and maintains is_duplicate
  as derived state so existing badges/counts keep working. Chained
  after scan_all_source_roots with a 60s countdown.

API
- GET /library/duplicates/groups returns all groups with members,
  bucketed in Python from one query. Each group has a reason ("exact"
  iff every member shares a SHA-256, "similar" otherwise).
- POST /library/maintenance/{regroup-duplicates,backfill-phashes}.

Frontend
- New DuplicatesView (sectioned grid, one section per cluster) replaces
  the timeline when the user is in the duplicates section. Each section
  shows a "Keep best, discard N" button that picks the highest-pixel
  copy and reuses the existing undoable bulk-discard so Cmd+Z works.
- Manual best override: hover any non-best thumbnail and click "Keep
  this" (Crown icon, top-right) to override the auto-pick. The header
  annotates "(manual)" so it's obvious which copy will be kept.
- Keyboard nav within the duplicates view walks the flat member list,
  with ↑/↓ jumping by the measured column count and scrollIntoView on
  every move. Timeline's keyboard handler now early-returns in the
  duplicates section so the two don't fight.
- BEST pill / Keep-this button live at top-right with a ring outline so
  they don't collide visually with the cyan selection ring around a
  selected cell. Dimensions chip moved to bottom-left to free both
  right corners for the keep affordances.
- New "Duplicates" section in SettingsDialog: shows group/member counts
  and exposes both backfill + re-detect actions, sharing a query cache
  with DuplicatesView via DUPLICATE_GROUPS_QUERY_KEY.
- PhotoInfoPanel "Basic Info" section now shows the photo's full file
  path in monospace below the size/dimensions/date grid.
- New imagehash==4.3.1 dep in requirements.txt.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/services/api.ts

@@ -349,7 +349,61 @@ export const library = {
       const response = await api.post('/library/maintenance/cleanup')
       return response.data
     },
+
+    /** Recompute duplicate groups from current perceptual hashes.
+     *  Idempotent — safe to fire repeatedly. */
+    regroupDuplicates: async (): Promise<{ status: string; message?: string }> => {
+      const response = await api.post('/library/maintenance/regroup-duplicates')
+      return response.data
+    },
+
+    /** Compute pHash for every photo currently missing one. One-shot
+     *  recovery path for libraries that existed before the phash column
+     *  was added. */
+    backfillPhashes: async (): Promise<{ status: string; message?: string }> => {
+      const response = await api.post('/library/maintenance/backfill-phashes')
+      return response.data
+    },
   },
+
+  /** Duplicate groups computed by app.services.duplicates.regroup_duplicates.
+   *  Drives the grouped grid view in the Duplicates section. */
+  duplicates: {
+    groups: async (): Promise<DuplicateGroupsResponse> => {
+      const response = await api.get('/library/duplicates/groups')
+      return response.data
+    },
+  },
+}
+
+// ── Duplicate groups ─────────────────────────────────────────────────────
+
+export interface DuplicateGroupMember {
+  id: string
+  filename: string
+  taken_at: string | null
+  file_size: number | null
+  width: number | null
+  height: number | null
+  thumb_small: string | null
+  file_hash: string | null
+  folder_id: string | null
+  media_type: string
+}
+
+export interface DuplicateGroup {
+  group_id: string
+  member_count: number
+  /** "exact" iff every member shares the same SHA-256 (true byte
+   *  duplicates that pHash also caught). "similar" otherwise. */
+  reason: 'exact' | 'similar'
+  members: DuplicateGroupMember[]
+}
+
+export interface DuplicateGroupsResponse {
+  groups: DuplicateGroup[]
+  total_groups: number
+  total_members: number
 }
 
 export interface LibraryStats {