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/components/dialogs/SettingsDialog.tsx

@@ -11,6 +11,8 @@ import {
   Cpu,
   AlertCircle,
   CheckCircle2,
+  Copy,
+  Sparkles,
 } from 'lucide-react'
 import clsx from 'clsx'
 import { useQuery, useQueryClient } from '@tanstack/react-query'
@@ -27,6 +29,9 @@ const SETTINGS_THUMB_STATS_KEY = ['settings', 'thumbnail-stats'] as const
 const SETTINGS_LIB_STATS_KEY = ['settings', 'library-stats'] as const
 const SETTINGS_WORKER_STATUS_KEY = ['settings', 'worker-status'] as const
 const SETTINGS_MISSING_STATS_KEY = ['settings', 'missing-stats'] as const
+// Shared with the DuplicatesView so a regroup invalidates the same cache
+// the grid renders from. Imported via the canonical hook key.
+import { DUPLICATE_GROUPS_QUERY_KEY } from '../../hooks/useDuplicateGroupsQuery'
 
 interface SettingsDialogProps {
   isOpen: boolean
@@ -86,6 +91,14 @@ export function SettingsDialog({ isOpen, onClose }: SettingsDialogProps) {
     refetchInterval: isOpen ? 5000 : false,
     staleTime: 0,
   })
+  // Duplicates: shares the cache with DuplicatesView so a regroup
+  // triggered from Settings updates the grid view immediately.
+  const duplicatesQuery = useQuery({
+    queryKey: DUPLICATE_GROUPS_QUERY_KEY,
+    queryFn: library.duplicates.groups,
+    enabled: isOpen,
+    staleTime: 0,
+  })
 
   const thumbStats = thumbStatsQuery.data
   const libStats = libStatsQuery.data
@@ -103,6 +116,7 @@ export function SettingsDialog({ isOpen, onClose }: SettingsDialogProps) {
   const refreshStats = useCallback(() => {
     queryClient.invalidateQueries({ queryKey: SETTINGS_THUMB_STATS_KEY })
     queryClient.invalidateQueries({ queryKey: SETTINGS_LIB_STATS_KEY })
+    queryClient.invalidateQueries({ queryKey: DUPLICATE_GROUPS_QUERY_KEY })
   }, [queryClient])
   const refreshWorkers = useCallback(() => {
     queryClient.invalidateQueries({ queryKey: SETTINGS_WORKER_STATUS_KEY })
@@ -249,6 +263,60 @@ export function SettingsDialog({ isOpen, onClose }: SettingsDialogProps) {
               </div>
             </Section>
 
+            {/* ----------------------------------------------------- */}
+            {/* Duplicate detection                                    */}
+            {/* ----------------------------------------------------- */}
+            <Section
+              icon={<Copy className="h-4 w-4" />}
+              title="Duplicates"
+            >
+              <div className="grid grid-cols-2 gap-2 text-xs">
+                <Stat
+                  label="Groups"
+                  value={duplicatesQuery.data?.total_groups}
+                />
+                <Stat
+                  label="Members"
+                  value={duplicatesQuery.data?.total_members}
+                />
+              </div>
+              <p className="mt-3 text-xs text-text-muted">
+                Duplicates are detected by perceptual hash (pHash), which catches
+                visually-identical photos even when their bytes differ — re-encoded
+                JPEGs, screenshots, resized exports. Backfill computes hashes for
+                photos that existed before pHash was added; Re-detect re-runs
+                clustering across the whole library.
+              </p>
+              <div className="mt-2 flex flex-wrap gap-2">
+                <ActionButton
+                  loading={busy['backfill-phashes']}
+                  onClick={() =>
+                    runAction(
+                      'backfill-phashes',
+                      () => library.maintenance.backfillPhashes(),
+                      'pHash backfill queued'
+                    )
+                  }
+                >
+                  <Sparkles className="h-4 w-4" />
+                  Backfill perceptual hashes
+                </ActionButton>
+                <ActionButton
+                  loading={busy['regroup-duplicates']}
+                  onClick={() =>
+                    runAction(
+                      'regroup-duplicates',
+                      () => library.maintenance.regroupDuplicates(),
+                      'Duplicate detection queued'
+                    )
+                  }
+                >
+                  <Copy className="h-4 w-4" />
+                  Re-detect duplicates
+                </ActionButton>
+              </div>
+            </Section>
+
             {/* ----------------------------------------------------- */}
             {/* Thumbnail maintenance                                  */}
             {/* ----------------------------------------------------- */}