Bug: delete_files return type annotation is wrong

[gkorland]

Description

In api/graph.py:408, the delete_files method is annotated as returning a tuple but actually returns None:

def delete_files(self, files: list[Path]) -> tuple[str, dict, list[int]]:
    # ... query logic ...
    return None

Impact

Type checkers and callers relying on the return type annotation will be misled. Any code unpacking the return value (e.g., a, b, c = graph.delete_files(...)) will fail at runtime.

Suggested Fix

Either fix the return type to -> None, or return the expected tuple.

Context

Found during code review of PR #522.