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/alembic/versions/0007_folder_hidden.py

@@ -0,0 +1,67 @@
+"""folders + photos is_hidden flag
+
+Revision ID: 0007_folder_hidden
+Revises: 0006_face_512d
+Create Date: 2026-04-11
+
+Adds an "exclude from cross-cutting views" flag:
+
+  folders.is_hidden   — user-toggled on a folder or source root. When
+                        true, photos in that subtree are hidden from
+                        library-wide views (All Photos, Map, Tags,
+                        People, Search, Duplicates, sidebar counts) but
+                        remain indexed and visible when the user
+                        navigates into the folder directly.
+
+  photos.is_hidden    — denormalized: true iff any ancestor folder in
+                        the photo's folder chain has is_hidden=true.
+                        Kept as a real column (rather than a recursive
+                        query per read) because the filter runs on
+                        essentially every photo query in the app, and
+                        the toggle operation that recomputes it is
+                        rare. Indexed so `WHERE NOT is_hidden` doesn't
+                        fall off the rating/taken_at indexes.
+
+Both columns default to false so existing rows need no backfill.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0007_folder_hidden"
+down_revision: Union[str, None] = "0006_face_512d"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "folders",
+        sa.Column(
+            "is_hidden",
+            sa.Boolean(),
+            nullable=False,
+            server_default=sa.false(),
+        ),
+    )
+    op.add_column(
+        "photos",
+        sa.Column(
+            "is_hidden",
+            sa.Boolean(),
+            nullable=False,
+            server_default=sa.false(),
+        ),
+    )
+    op.create_index(
+        "ix_photos_is_hidden",
+        "photos",
+        ["is_hidden"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_photos_is_hidden", table_name="photos")
+    op.drop_column("photos", "is_hidden")
+    op.drop_column("folders", "is_hidden")