feat: real recursive folder tree in the sidebar

The Folders section in the LeftSidebar previously rendered the flat
list of source roots — actual subdirectories were invisible. Now it
shows the full nested tree, click any node to filter, drop targets
work at every depth.

Backend
- New GET /folders/tree returning a list of root nodes (one per
  active SourceRoot). Each node is { id, name, path, photo_count,
  children: [...] } with children sorted alphabetically at every
  level. Walks Folder rows whose source_root_id matches and whose
  path is at or beneath the source root, then attaches them by
  parent path so partial scans don't break the tree.
- The source root's display label is overlaid on the root folder
  node so the top-level entry reads as "Library" instead of
  "/photos".
- list_photos folder_id filter now does descendant matching: when
  a Folder id is given, it includes the folder itself and every
  Folder whose path is a sep-prefixed descendant. Matches the
  Lightroom mental model: clicking "Library" or any parent folder
  shows everything beneath it. The existing source-root-id branch
  is unchanged.

Frontend
- New types/api.ts FolderTreeNode interface and sourceFolders.tree()
  helper.
- New hooks/useFolderTreeQuery.ts with a 30s staleTime and a
  findFolderInTree() walker for id-based name lookups.
- LeftSidebar drops the flat foldersData list and uses the tree
  query. folderNodeToTreeItem recursively maps backend nodes into
  the existing TreeItem shape; renderTreeItem already knew how to
  recurse into children, so the tree just works at any depth.
  Drop targets, drag-to-move, drag-to-copy, double-click rename,
  and active-state highlighting all carry over to nested folders.
- The renameMutation now also invalidates ['folders', 'tree'] so a
  source-root rename refreshes the tree label immediately.
- ActiveFilterChips switches to the tree query and uses the new
  findFolderInTree walker so the chip label resolves correctly for
  sub-folder filters too — not just top-level source roots.
- The "Scan all folders" button visibility now keys off the tree
  length instead of the flat folders length.

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

backend/app/routers/photos.py

@@ -71,15 +71,17 @@ async def list_photos(
     if date_to:
         filters.append(Photo.taken_at <= date_to)
     
-    # Folder filter — the sidebar exposes "source roots" (top-level scan
-    # paths) under the same UI affordance as folders, so the same param has
-    # to accept either a folder id or a source root id. If the value matches
-    # a source root, expand to every folder under that root and use IN.
+    # Folder filter. The sidebar can pass either a SourceRoot id or a
+    # Folder id; both should include descendants so clicking a parent
+    # folder shows everything under it (Lightroom semantics).
     if folder_id:
         sr_check = await db.execute(
-            select(SourceRoot.id).where(SourceRoot.id == folder_id)
+            select(SourceRoot).where(SourceRoot.id == folder_id)
         )
-        if sr_check.scalar_one_or_none() is not None:
+        sr_row = sr_check.scalar_one_or_none()
+
+        if sr_row is not None:
+            # Source root → all folders under it (any depth).
             child_folders = await db.execute(
                 select(Folder.id).where(Folder.source_root_id == folder_id)
             )
@@ -87,11 +89,25 @@ async def list_photos(
             if child_ids:
                 filters.append(Photo.folder_id.in_(child_ids))
             else:
-                # Source root with no folder rows yet — match nothing rather
-                # than returning the entire library.
                 filters.append(Photo.id == '__no_match__')
         else:
-            filters.append(Photo.folder_id == folder_id)
+            # Folder id → that folder + every descendant by path prefix.
+            target_check = await db.execute(
+                select(Folder).where(Folder.id == folder_id)
+            )
+            target = target_check.scalar_one_or_none()
+            if target is None:
+                filters.append(Photo.id == '__no_match__')
+            else:
+                target_path = os.path.normpath(target.path).rstrip(os.sep)
+                desc_result = await db.execute(
+                    select(Folder.id).where(
+                        (Folder.path == target_path)
+                        | (Folder.path.like(target_path + os.sep + '%'))
+                    )
+                )
+                desc_ids = [row[0] for row in desc_result.all()]
+                filters.append(Photo.folder_id.in_(desc_ids))
     
     # Media type filter
     if media_type: