feat: wire frontend to vision pipeline search and tags

- Add search API client (POST /photos/search) and useSearchQuery hook
  for hybrid FTS + semantic search with RRF ranking
- Extend Tag type with kind, source, representative_photo_id fields
- Add tags.merge() API method
- Update useTagsQuery to accept optional kind filter
- Add People section to sidebar (face clusters from GET /tags?kind=face_cluster)
- Sidebar Tags count now shows user tags only; People shows face clusters

The existing GET /photos?q= flow is preserved for browsing; the new
search hook activates when the search box has a non-empty query.

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

frontend/src/components/layout/LeftSidebar.tsx

@@ -17,6 +17,7 @@ import {
   Pencil,
   PanelLeftClose,
   Settings,
+  Users,
 } from 'lucide-react'
 import clsx from 'clsx'
 import { sourceFolders, photos as photosApi, type FolderTreeNode } from '../../services/api'
@@ -63,6 +64,7 @@ export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
   const navigateToSection = useFilterStore((s) => s.navigateToSection)
   const currentSection = useFilterStore((s) => s.currentSection)
   const { data: allTags = [] } = useTagsQuery()
+  const { data: faceClusters = [] } = useTagsQuery('face_cluster')
   const { data: stats } = useLibraryStatsQuery()
   const [dropTargetId, setDropTargetId] = useState<string | null>(null)
 
@@ -255,6 +257,9 @@ export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
       case 'tags':
         navigateToSection('tags', { groupBy: 'tag' })
         break
+      case 'people':
+        navigateToSection('people', { groupBy: 'tag' })
+        break
       case 'colors':
         navigateToSection('colors', { groupBy: 'color' })
         break
@@ -349,8 +354,10 @@ export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
       : undefined,
   })
 
-  // Total tag count for the badge on the Tags entry.
-  const tagsTotalCount = allTags.reduce((sum, t) => sum + (t.photo_count || 0), 0)
+  // Total tag count for the badge on the Tags entry (user tags only).
+  const userTags = allTags.filter((t) => t.kind === 'user')
+  const tagsTotalCount = userTags.reduce((sum, t) => sum + (t.photo_count || 0), 0)
+  const peopleTotalCount = faceClusters.reduce((sum, t) => sum + (t.photo_count || 0), 0)
 
   const libraryTree: TreeItem[] = [
     {
@@ -361,6 +368,7 @@ export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
         { id: 'all-photos', label: 'All Photos', icon: <Image className="h-4 w-4" />, count: stats?.all_photos ?? 0 },
         { id: 'rated', label: 'Rated', icon: <Star className="h-4 w-4" />, count: stats?.rated ?? 0 },
         { id: 'tags', label: 'Tags', icon: <TagIcon className="h-4 w-4" />, count: tagsTotalCount },
+        { id: 'people', label: 'People', icon: <Users className="h-4 w-4" />, count: peopleTotalCount },
         { id: 'colors', label: 'Colors', icon: <Palette className="h-4 w-4" />, count: stats?.colored ?? 0 },
         { id: 'map', label: 'Map', icon: <MapPin className="h-4 w-4" />, count: stats?.with_gps ?? 0 },
         { id: 'duplicates', label: 'Duplicates', icon: <Copy className="h-4 w-4" />, count: stats?.duplicates ?? 0 },