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/hooks/useSearchQuery.ts

@@ -0,0 +1,37 @@
+import { useQuery } from '@tanstack/react-query'
+import { search, type SearchResult } from '../services/api'
+import { useFilterStore } from '../store/filterStore'
+
+/**
+ * Hybrid search hook — fires POST /photos/search when the user has a
+ * non-empty search query. Returns results ranked by RRF (FTS + semantic).
+ *
+ * When `q` is empty, this hook is disabled and returns no data — the
+ * normal usePhotosQuery takes over for browse mode.
+ */
+export function useSearchQuery() {
+  const q = useFilterStore((s) => s.q)
+  const tagIds = useFilterStore((s) => s.tagIds)
+  const dateFrom = useFilterStore((s) => s.dateFrom)
+  const dateTo = useFilterStore((s) => s.dateTo)
+
+  const hasQuery = q.trim().length > 0
+
+  return useQuery<SearchResult[]>({
+    queryKey: ['search', q, tagIds, dateFrom, dateTo],
+    queryFn: async () => {
+      const resp = await search.query({
+        q: q.trim(),
+        filters: {
+          tag_ids: tagIds.length > 0 ? tagIds : undefined,
+          date_from: dateFrom ?? undefined,
+          date_to: dateTo ?? undefined,
+        },
+        limit: 200,
+      })
+      return resp.results
+    },
+    enabled: hasQuery,
+    staleTime: 30_000,
+  })
+}

frontend/src/hooks/useTagsQuery.ts

@@ -1,12 +1,12 @@
 import { useQuery } from '@tanstack/react-query'
-import { tags as tagsApi, type Tag } from '../services/api'
+import { tags as tagsApi, type Tag, type TagKind } from '../services/api'
 
 export const TAGS_QUERY_KEY = ['tags'] as const
 
-export function useTagsQuery() {
+export function useTagsQuery(kind?: TagKind) {
   return useQuery<Tag[]>({
-    queryKey: TAGS_QUERY_KEY,
-    queryFn: tagsApi.list,
+    queryKey: kind ? ['tags', kind] : TAGS_QUERY_KEY,
+    queryFn: () => tagsApi.list(kind),
     staleTime: 30_000,
   })
 }