fix: resolve all 28 mypy type errors + re-enable coverage in CI

backend/indexer.py

@@ -5,7 +5,7 @@ import re
 import threading
 from pathlib import Path
 from datetime import datetime, timezone
-from typing import Dict, List, Optional, Any
+from typing import Any, Callable, Dict, List, Optional
 
 import frontmatter
 
@@ -21,14 +21,14 @@ vault_config: Dict[str, Dict[str, Any]] = {}
 _index_lock = threading.Lock()
 
 # Async lock for partial index updates (coexists with threading lock)
-_async_index_lock: asyncio.Lock = None  # initialized lazily
+_async_index_lock: asyncio.Lock | None = None  # initialized lazily
 
 # Generation counter — incremented on each index rebuild so consumers
 # (e.g. the inverted index in search.py) can detect staleness.
 _index_generation: int = 0
 
 # Hook for incremental inverted index updates: called as (action, vault, path, file_info)
-_on_index_change: callable = None
+_on_index_change: Callable[..., None] | None = None
 
 
 def set_index_change_hook(hook):
@@ -615,7 +615,7 @@ def _remove_file_from_structures(vault_name: str, rel_path: str) -> Optional[Dic
 
     # Notify inverted index for incremental update
     if _on_index_change:
-        _on_index_change('remove', vault_name, rel_path, removed)
+        _on_index_change('remove', vault_name, rel_path, removed)  # type: ignore[misc]
 
     return removed
 
@@ -686,7 +686,7 @@ def _add_file_to_structures(vault_name: str, file_info: Dict[str, Any]):
 
     # Notify inverted index for incremental update
     if _on_index_change:
-        _on_index_change('add', vault_name, file_info["path"], file_info)
+        _on_index_change('add', vault_name, file_info["path"], file_info)  # type: ignore[misc]
 
 
 async def update_single_file(vault_name: str, abs_file_path: str) -> Optional[Dict[str, Any]]: