fix: admin scan skips other users' source root directories

When the admin's source root is the mount root (/photos) and other
users have subdirectories (/photos/bob), the admin's scan now prunes
those directories from os.walk so photos aren't double-indexed under
the wrong user. The scanner queries all active source roots owned by
other users and excludes their paths during directory traversal.

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

backend/app/tasks/scan.py

@@ -164,11 +164,28 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
                 hidden_folder_cache[folder_row.id] = False
                 return False
 
+            # Build a set of paths owned by OTHER users' source roots so
+            # the admin's scan of /photos doesn't descend into /photos/bob.
+            other_root_paths: set[str] = set()
+            if owner_user_id:
+                other_roots = (await session.execute(
+                    select(SourceRoot.path)
+                    .where(SourceRoot.is_active == True)  # noqa: E712
+                    .where(SourceRoot.user_id != owner_user_id)
+                )).scalars().all()
+                other_root_paths = {os.path.normpath(p) for p in other_roots}
+
+            def _should_skip_dir(dirpath: str) -> bool:
+                """True if dirpath is another user's source root."""
+                return os.path.normpath(dirpath) in other_root_paths
+
             # Pre-walk to compute the total file count upfront. Without this
             # the progress bar would jump every time a new subfolder is
             # encountered because the running total kept growing.
             total_files = 0
             for _root, _dirs, files in os.walk(folder_path):
+                # Prune other users' directories from count
+                _dirs[:] = [d for d in _dirs if not _should_skip_dir(os.path.join(_root, d))]
                 total_files += sum(
                     1 for f in files if Path(f).suffix.lower() in SUPPORTED_EXTENSIONS
                 )
@@ -179,6 +196,9 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
             errors = []
 
             for root, dirs, files in os.walk(folder_path):
+                # Skip directories that belong to other users' source roots
+                dirs[:] = [d for d in dirs if not _should_skip_dir(os.path.join(root, d))]
+
                 # Get or create folder entry
                 folder = await get_or_create_folder(session, root, source_root_id, owner_user_id)
                 progress_set(REDIS_KEY_CURRENT_FOLDER, root)