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/App.tsx

@@ -1,5 +1,6 @@
 import { useState } from 'react'
 import { Timeline } from './components/timeline/Timeline'
+import { DuplicatesView } from './components/duplicates/DuplicatesView'
 import { LeftSidebar } from './components/layout/LeftSidebar'
 import { RightSidebar } from './components/layout/RightSidebar'
 import { TopBar } from './components/layout/TopBar'
@@ -12,6 +13,7 @@ import { FilterBar } from './components/filter/FilterBar'
 import { DiscardActionBar } from './components/discard/DiscardActionBar'
 import { SettingsDialog } from './components/dialogs/SettingsDialog'
 import { usePhotoStore } from './store/photoStore'
+import { useFilterStore } from './store/filterStore'
 import { useKeyboardShortcuts } from './hooks/useKeyboardShortcuts'
 import { useFilterUrlSync } from './hooks/useFilterUrlSync'
 import { usePhotosQuery } from './hooks/usePhotosQuery'
@@ -21,6 +23,7 @@ function App() {
   const [rightSidebarOpen, setRightSidebarOpen] = useState(true)
   const [settingsOpen, setSettingsOpen] = useState(false)
   const viewMode = usePhotoStore((state) => state.viewMode)
+  const currentSection = useFilterStore((s) => s.currentSection)
 
   // Bidirectional sync of filter store with URL query params.
   useFilterUrlSync()
@@ -71,7 +74,11 @@ function App() {
           <FilterBar />
           <DiscardActionBar />
           <div className="flex-1 overflow-auto">
-            <Timeline />
+            {/* The Duplicates section gets its own grouped grid view —
+             *  the regular timeline can't represent groups, and a flat
+             *  filtered list of "is_duplicate=true" photos was the old
+             *  half-broken UX. */}
+            {currentSection === 'duplicates' ? <DuplicatesView /> : <Timeline />}
           </div>
           {/* Floating keyboard hints — bottom-center of the main column,
            *  glassy. Mounted here so it's centered against the timeline,