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/services/api.ts

@@ -519,21 +519,27 @@ export const heaps = {
 }
 
 // Tags API
+export type TagKind = 'user' | 'object' | 'scene' | 'face_cluster'
+
 export interface Tag {
   id: string
   name: string
   color: string | null
+  kind: TagKind
+  source: string | null
+  representative_photo_id: string | null
   photo_count: number
 }
 
 export const tags = {
-  list: async (): Promise<Tag[]> => {
-    const response = await api.get('/tags')
+  list: async (kind?: TagKind): Promise<Tag[]> => {
+    const params = kind ? { kind } : undefined
+    const response = await api.get('/tags', { params })
     return response.data
   },
 
-  create: async (name: string, color?: string): Promise<Tag> => {
-    const response = await api.post('/tags', { name, color })
+  create: async (name: string, color?: string, kind: TagKind = 'user'): Promise<Tag> => {
+    const response = await api.post('/tags', { name, color, kind })
     return response.data
   },
 
@@ -546,6 +552,11 @@ export const tags = {
     await api.delete(`/tags/${tagId}`)
   },
 
+  merge: async (sourceId: string, targetId: string): Promise<{ merged_into: string; target_name: string }> => {
+    const response = await api.post(`/tags/${sourceId}/merge`, { target_id: targetId })
+    return response.data
+  },
+
   /** Add one or more tags to a photo. */
   addToPhoto: async (photoId: string, tagIds: string[]) => {
     const response = await api.post(`/photos/${photoId}/tags`, { tag_ids: tagIds })
@@ -558,6 +569,38 @@ export const tags = {
   },
 }
 
+// Search API — hybrid FTS + semantic search
+export interface SearchResult {
+  id: string
+  filename: string
+  filepath: string
+  media_type: string
+  width: number
+  height: number
+  taken_at: string | null
+  rating: number
+  color_label: string | null
+  thumb_small: string
+  thumb_medium: string
+  score: number
+}
+
+export const search = {
+  query: async (params: {
+    q?: string
+    filters?: {
+      tag_ids?: string[]
+      date_from?: string
+      date_to?: string
+    }
+    limit?: number
+    offset?: number
+  }): Promise<{ results: SearchResult[]; total: number }> => {
+    const response = await api.post('/photos/search', params)
+    return response.data
+  },
+}
+
 // Discard API
 export const discard = {
   list: async () => {