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/services/api.ts

@@ -191,32 +191,43 @@ export const heaps = {
 }
 
 // Tags API
+export interface Tag {
+  id: string
+  name: string
+  color: string | null
+  photo_count: number
+}
+
 export const tags = {
-  list: async () => {
+  list: async (): Promise<Tag[]> => {
     const response = await api.get('/tags')
     return response.data
   },
 
-  create: async (name: string, color?: string) => {
-    const response = await api.post('/tags', {
-      name,
-      color,
-    })
+  create: async (name: string, color?: string): Promise<Tag> => {
+    const response = await api.post('/tags', { name, color })
     return response.data
   },
 
-  update: async (tagId: string, data: {
-    name?: string
-    color?: string
-  }) => {
+  update: async (tagId: string, data: { name?: string; color?: string }): Promise<Tag> => {
     const response = await api.patch(`/tags/${tagId}`, data)
     return response.data
   },
 
-  delete: async (tagId: string) => {
-    const response = await api.delete(`/tags/${tagId}`)
+  delete: async (tagId: string): Promise<void> => {
+    await api.delete(`/tags/${tagId}`)
+  },
+
+  /** Add one or more tags to a photo. */
+  addToPhoto: async (photoId: string, tagIds: string[]) => {
+    const response = await api.post(`/photos/${photoId}/tags`, { tag_ids: tagIds })
     return response.data
   },
+
+  /** Remove a tag from a photo. */
+  removeFromPhoto: async (photoId: string, tagId: string): Promise<void> => {
+    await api.delete(`/photos/${photoId}/tags/${tagId}`)
+  },
 }
 
 // Discard API