Add granular DB error types with retry logic

New exceptions: DatabaseBusyError, DatabasePermissionDeniedError,
DatabasePathInvalidError, DatabaseCorruptedError, DatabaseUnavailableError.

open_db creates parent directory if missing. Catches all aiosqlite errors
and maps to specific exception types.

get_db retries up to 3x on locked database with backoff.
Propagates specific exceptions instead of generic HTTPException.

Tests for all new error types and retry behavior.

backend/app/db.py

@@ -475,14 +475,75 @@ async def init_db(db: aiosqlite.Connection) -> None:
 async def open_db(database_path: str) -> aiosqlite.Connection:
     """Open a new application SQLite connection with the standard settings.
 
+    Creates the parent directory if it does not exist.
+
     Args:
         database_path: Path to the BanGUI SQLite database.
 
     Returns:
         A configured :class:`aiosqlite.Connection` instance.
+
+    Raises:
+        DatabasePathInvalidError: If the directory cannot be created or is inaccessible.
+        DatabasePermissionDeniedError: If aiosqlite.connect raises PermissionError.
+        DatabaseCorruptedError: If the database file is corrupted.
+        DatabaseUnavailableError: For any other unexpected error.
     """
-    await _cleanup_wal_files(database_path)
-    db = await aiosqlite.connect(database_path)
+    from app.exceptions import (
+        DatabaseCorruptedError,
+        DatabasePathInvalidError,
+        DatabasePermissionDeniedError,
+        DatabaseUnavailableError,
+    )
+
+    db_dir = Path(database_path).parent
+    if not db_dir.exists():
+        try:
+            db_dir.mkdir(parents=True, exist_ok=True)
+        except PermissionError as exc:
+            log.error("database_open_failed", error=str(exc), database_path=database_path)
+            raise DatabasePathInvalidError(database_path) from exc
+        except OSError as exc:
+            log.error("database_open_failed", error=str(exc), database_path=database_path)
+            raise DatabaseUnavailableError(database_path, str(exc)) from exc
+
+    try:
+        db = await aiosqlite.connect(database_path)
+    except PermissionError as exc:
+        log.error("database_open_failed", error=str(exc), database_path=database_path)
+        raise DatabasePermissionDeniedError(database_path) from exc
+    except aiosqlite.OperationalError as exc:
+        error_msg = str(exc).lower()
+        sqlite_code = getattr(exc, "sqlite_errorcode", None)
+        log.error(
+            "database_open_failed",
+            error=str(exc),
+            sqlite_errorcode=sqlite_code,
+            database_path=database_path,
+        )
+        if "database is locked" in error_msg or "busy" in error_msg:
+            raise DatabaseUnavailableError(database_path, str(exc)) from exc
+        if "unable to open database file" in error_msg:
+            raise DatabasePathInvalidError(database_path) from exc
+        raise DatabaseUnavailableError(database_path, str(exc)) from exc
+    except aiosqlite.DatabaseError as exc:
+        log.error(
+            "database_open_failed",
+            error=str(exc),
+            database_path=database_path,
+        )
+        raise DatabaseCorruptedError(database_path) from exc
+    except OSError as exc:
+        log.error("database_open_failed", error=str(exc), database_path=database_path)
+        raise DatabaseUnavailableError(database_path, str(exc)) from exc
+    except Exception as exc:
+        log.error("database_open_failed", error=str(exc), database_path=database_path)
+        raise DatabaseUnavailableError(database_path, str(exc)) from exc
+
     db.row_factory = aiosqlite.Row
-    await _configure_connection(db)
+    try:
+        await _configure_connection(db)
+    except Exception:
+        await db.close()
+        raise
     return db