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.
