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

backend/attachment_indexer.py

@@ -1,7 +1,7 @@
 import asyncio
 import logging
 from pathlib import Path
-from typing import Dict, List, Optional
+from typing import Any, Dict, List, Optional
 import threading
 
 logger = logging.getLogger("obsigate.attachment_indexer")
@@ -34,7 +34,7 @@ def clear_resolution_cache(vault_name: Optional[str] = None) -> None:
                 del _resolution_cache[key]
 
 
-def _scan_vault_attachments(vault_name: str, vault_path: str, vault_cfg: dict = None) -> Dict[str, List[Path]]:
+def _scan_vault_attachments(vault_name: str, vault_path: str, vault_cfg: dict | None = None) -> Dict[str, List[Path]]:
     """Synchronously scan a vault directory for image attachments.
     
     Walks the vault tree and builds a filename -> absolute path mapping
@@ -84,7 +84,7 @@ def _scan_vault_attachments(vault_name: str, vault_path: str, vault_cfg: dict =
     return index
 
 
-async def build_attachment_index(vault_config: Dict[str, Dict[str, any]]) -> None:
+async def build_attachment_index(vault_config: Dict[str, Dict[str, Any]]) -> None:
     """Build the attachment index for all configured vaults.
     
     Runs vault scans concurrently in a thread pool, then performs
@@ -103,7 +103,7 @@ async def build_attachment_index(vault_config: Dict[str, Dict[str, any]]) -> Non
     loop = asyncio.get_event_loop()
     new_index: Dict[str, Dict[str, List[Path]]] = {}
     
-    tasks = []
+    tasks: list[tuple[str, asyncio.Future[Dict[str, List[Path]]]]] = []
     for name, config in vault_config.items():
         vault_path = config.get("path")
         if not vault_path:
@@ -132,7 +132,7 @@ async def build_attachment_index(vault_config: Dict[str, Dict[str, any]]) -> Non
     logger.info(f"Attachment index built: {len(attachment_index)} vaults, {total_attachments} total attachments")
 
 
-async def rescan_vault_attachments(vault_name: str, vault_path: str, vault_cfg: dict = None) -> int:
+async def rescan_vault_attachments(vault_name: str, vault_path: str, vault_cfg: dict | None = None) -> int:
     """Rescan attachments for a single vault.
     
     Args: