feat: hide-from-views flag on folders

Adds a per-folder "hide from views" toggle so noisy subtrees
(screenshots, WhatsApp dumps, work archives) can be excluded from
cross-cutting views without losing indexing. Photos under a hidden
folder are still scanned, thumbnailed, embedded, OCR'd, face-
extracted — they just stop appearing in All Photos, Rated, Colors,
Map, Tags, People, Search, Duplicates, and the sidebar counts.
Navigating directly into the folder still shows every photo.

Schema (migration 0007_folder_hidden):
- folders.is_hidden   user-set toggle, default false
- photos.is_hidden    denormalized effective flag (true iff any
                      ancestor folder is hidden), indexed so cross-
                      cutting queries stay on the existing planner
                      paths

The denorm is maintained by two paths:
- The scanner walks the ancestry chain on insert, with a per-scan
  memoized cache so each folder is resolved once per scan.
- POST /api/v1/folders/{id}/hide flips folders.is_hidden and runs a
  WITH RECURSIVE CTE to recompute every folder's effective state in
  one query, then bulk-updates photos WHERE IS DISTINCT FROM. Runs
  in ~10 ms on a 13k-photo library.

Filters added (cross-cutting queries):
- /library/stats — every sidebar badge via a shared `visible` filter
- /photos (list) — only when neither folder_id nor heap_id is set;
  folder browse and heap browse always show everything
- /photos/map
- /library/duplicates/groups
- /folders/tree photo_count subquery
- /tags count_subq (drives Tags + People sidebar counts)
- services/duplicates.regroup_duplicates (so hidden dupes never
  contaminate the Duplicates view)
- services/search.hybrid_search — both semantic (pgvector) and FTS
  legs join photos so rankings don't include hidden results

Intentionally NOT filtered:
- /photos?folder_id=X and /photos?heap_id=X (user-intentional browse)
- /library/maintenance/pipeline-stats (tracks real worker state)
- cleanup service (disk-level ops, not views)

Frontend:
- sourceFolders.setHidden(id, hidden) API client method
- FolderTreeNode.is_hidden carried through the tree into TreeItem
- LeftSidebar kebab menu: "Hide from views" / "Show in views" with a
  mutation that invalidates folders, photos, stats, and tags caches
- Hidden folder rows swap the Folder icon for EyeOff and render the
  label italic/muted so the state is visible at a glance

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

backend/app/routers/photos.py

@@ -132,6 +132,19 @@ async def list_photos(
     # Discard filter — defaults to hiding discarded photos
     filters.append(Photo.is_discarded == is_discarded)
 
+    # Hidden-folder filter. Photos in folders the user has marked
+    # "hidden from views" (or any descendant of one) are excluded from
+    # every cross-cutting listing — All Photos, Rated, Colors, Tags,
+    # People, Map, search, etc. We only apply the filter when the
+    # request isn't already scoped to a user-intentional collection:
+    #   - folder_id set: the user is explicitly browsing that folder,
+    #     which is precisely how hidden folders are "opened" again.
+    #   - heap_id set: heaps are hand-curated. If the user added a
+    #     photo to a heap and later hid its folder, the heap still
+    #     reflects their explicit pick.
+    if not folder_id and not heap_id:
+        filters.append(Photo.is_hidden.is_(False))
+
     # Duplicate filter — only applied when explicitly set, so the default
     # view shows everything regardless of duplicate status.
     if is_duplicate is not None:
@@ -229,6 +242,7 @@ async def list_photos_with_gps(db: AsyncSession = Depends(get_db)):
             Photo.taken_at,
         ).where(
             Photo.is_discarded.is_(False),
+            Photo.is_hidden.is_(False),
             Photo.latitude.is_not(None),
             Photo.longitude.is_not(None),
         )