feat: tags end-to-end (CRUD, photo membership, filter, sidebar UI)

The Tag model and photo_tags join table were already in place; this
fills in the rest — full backend CRUD, per-photo add/remove, list-
endpoint filtering, and a Tags section in the RightSidebar with
autocomplete-create.

Backend
- routers/tags.py rewritten from a 27-line stub:
    GET    /tags                — list with photo counts
    POST   /tags                — create (idempotent on name)
    PATCH  /tags/{id}           — rename / recolor
    DELETE /tags/{id}           — delete (FK cascades photo_tags)
- routers/photos.py:
    POST   /photos/{id}/tags    — add tag ids (idempotent)
    DELETE /photos/{id}/tags/{tag_id} — remove
    GET    /photos/{id}         — now returns a `tags` list alongside
                                  the existing PhotoResponse fields
                                  (fetched via the photo_tags join)
- list_photos applies the existing tag_ids query param: comma-
  separated, AND semantics, one IN-subquery per id since SQLite
  has no native set-contains-all.

Frontend
- New hooks/useTagsQuery.ts.
- services/api.ts: Tag interface, full tags client (list/create/
  update/delete), addToPhoto/removeFromPhoto helpers.
- filterStore: tagIds: string[] field, setTagIds, toggleTagId,
  hasActiveFilters update, filtersToParams sends tag_ids comma list.
- useFilterUrlSync round-trips ?tag_ids=… so tag-filtered views
  are bookmarkable.
- usePhotosQuery threads tagIds through.
- RightSidebar gains a new Tags section using a TagsEditor
  component:
    - shows existing tag chips with X to remove
    - autocomplete input that matches the user's typing against
      existing tag names
    - shows an inline "+ Create '<name>'" affordance when there's
      no exact match
    - Enter creates and attaches in one shot; Esc clears the input
    - existing colour values render as a tinted chip background
- FilterBar gets a Tags group (only rendered when there's at
  least one tag) with toggleable chips per tag.
- ActiveFilterChips shows "Tag: <name>" chips for each active
  tag id, looking up names lazily from the tags query.

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

frontend/src/store/filterStore.ts

@@ -24,6 +24,8 @@ export interface FilterState {
   heapId: string | null
   /** When set, restrict to photos in this folder. */
   folderId: string | null
+  /** Restrict to photos that have ALL of these tag ids (AND semantics). */
+  tagIds: string[]
   sortBy: SortField
   sortOrder: SortOrder
 }
@@ -40,6 +42,8 @@ interface FilterStore extends FilterState {
   setFlag: (flag: FlagFilter) => void
   setHeapId: (id: string | null) => void
   setFolderId: (id: string | null) => void
+  setTagIds: (ids: string[]) => void
+  toggleTagId: (id: string) => void
   setSortBy: (field: SortField) => void
   setSortOrder: (order: SortOrder) => void
   toggleSortOrder: () => void
@@ -61,6 +65,7 @@ export const INITIAL_FILTERS: FilterState = {
   flag: 'any',
   heapId: null,
   folderId: null,
+  tagIds: [],
   sortBy: 'taken_at',
   sortOrder: 'desc',
 }
@@ -83,6 +88,13 @@ export const useFilterStore = create<FilterStore>((set) => ({
   setFlag: (flag) => set({ flag }),
   setHeapId: (heapId) => set({ heapId }),
   setFolderId: (folderId) => set({ folderId }),
+  setTagIds: (tagIds) => set({ tagIds }),
+  toggleTagId: (id) =>
+    set((s) => ({
+      tagIds: s.tagIds.includes(id)
+        ? s.tagIds.filter((t) => t !== id)
+        : [...s.tagIds, id],
+    })),
   setSortBy: (sortBy) => set({ sortBy }),
   setSortOrder: (sortOrder) => set({ sortOrder }),
   toggleSortOrder: () =>
@@ -108,6 +120,7 @@ export function filtersToParams(f: FilterState): Record<string, string | number>
   if (f.flag === 'discarded') params.is_discarded = 'true'
   if (f.heapId) params.heap_id = f.heapId
   if (f.folderId) params.folder_id = f.folderId
+  if (f.tagIds.length > 0) params.tag_ids = f.tagIds.join(',')
   params.sort = f.sortBy
   params.order = f.sortOrder
   return params
@@ -124,6 +137,7 @@ export function hasActiveFilters(f: FilterState): boolean {
     f.colorLabel !== null ||
     f.flag !== 'any' ||
     f.heapId !== null ||
-    f.folderId !== null
+    f.folderId !== null ||
+    f.tagIds.length > 0
   )
 }