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>

backend/app/models/photos.py

@@ -60,8 +60,24 @@ class Photo(Base):
     # photo just means adding it to the active heap. Both DB columns may still
     # exist on legacy installs but are no longer read or written.
     
-    # Duplicate detection
+    # Duplicate detection.
+    #
+    #   - file_hash (above): SHA-256 of the raw bytes. Catches byte-identical
+    #     copies but not visually-identical re-encodes / resizes / screenshots.
+    #   - phash: 16-char hex of a 64-bit perceptual hash, computed by the
+    #     thumbs worker from the decoded original frame. Robust to resize and
+    #     re-compression — this is what actually identifies "the same photo
+    #     saved twice with different JPEG quality".
+    #   - duplicate_group_id: shared by every photo in the same duplicate
+    #     cluster. Maintained by app.services.duplicates.regroup_duplicates,
+    #     not on individual writes — recomputed in batches after scans / on
+    #     demand from the Settings panel.
+    #   - is_duplicate: derived boolean (group_id IS NOT NULL). Kept as a real
+    #     column so the existing PhotoThumbnail badge and /library/stats
+    #     duplicates count don't have to change.
     is_duplicate = Column(Boolean, default=False)
+    phash = Column(String(16), index=True)
+    duplicate_group_id = Column(String, index=True)
     
     # Live photo support
     live_photo_video_id = Column(String, ForeignKey('photos.id'))