feat: tag-grouped timeline view

Reworks the Tags sidebar entry from an expandable list of tags into a
single leaf entry. Clicking it switches the timeline grouping mode to
"tag" — every tag becomes a sticky-headered group, with an "Untagged"
group at the bottom for photos with no tags. A photo with N tags
appears in N groups. Existing filters and sort still apply within each
group.

Backend
- list_photos eagerly loads Photo.tags via selectinload to avoid an
  N+1 round-trip.
- Each photo in the list response now carries a `tags: [{id, name,
  color}]` array. The route stops using PhotoListResponse strict
  validation (returns a plain dict with the same shape plus the new
  field) so we don't have to extend the pydantic schema.

Frontend
- Photo TS type gains an optional tags field plus a PhotoTagSummary
  alias.
- filterStore: new groupBy: 'date' | 'tag' field, default 'date',
  with setGroupBy + URL sync via ?group=tag. clearAll resets it.
- usePhotosQuery threads groupBy through filtersToParams (it's
  client-side only but kept in the params for cache key
  consistency).
- LeftSidebar Tags entry is now a leaf node (no children), shows the
  total tag photo count as the badge, and is highlighted when
  groupBy === 'tag'. Click → setGroupBy('tag') without touching
  other filters. Selecting "All Photos" resets groupBy back to
  'date' via clearAll.
- Timeline.buildItems gets a third "tag" branch:
  - Iterates photos × tags into per-tag buckets
  - Photos with no tags go into an "Untagged" bucket
  - Tag groups sorted alphabetically; Untagged pinned to the end
  - Headers + rows pushed in the same shape the date branch uses,
    so the existing sticky-header overlay works for free
- Selection state is by photo id, so a photo appearing in multiple
  groups stays consistently selected/highlighted across instances.

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

frontend/src/hooks/usePhotosQuery.ts

@@ -22,6 +22,7 @@ export function usePhotosQuery() {
   const folderId = useFilterStore((s) => s.folderId)
   const tagIds = useFilterStore((s) => s.tagIds)
   const duplicates = useFilterStore((s) => s.duplicates)
+  const groupBy = useFilterStore((s) => s.groupBy)
   const sortBy = useFilterStore((s) => s.sortBy)
   const sortOrder = useFilterStore((s) => s.sortOrder)
 
@@ -39,10 +40,11 @@ export function usePhotosQuery() {
         folderId,
         tagIds,
         duplicates,
+        groupBy,
         sortBy,
         sortOrder,
       }),
-    [q, dateFrom, dateTo, mediaTypes, ratingMin, colorLabel, flag, heapId, folderId, tagIds, duplicates, sortBy, sortOrder]
+    [q, dateFrom, dateTo, mediaTypes, ratingMin, colorLabel, flag, heapId, folderId, tagIds, duplicates, groupBy, sortBy, sortOrder]
   )
 
   return useQuery({