Implement attach_runs functionality for studies

[lucifer4330k]

Description

The attach_runs function in src/database/studies.py is currently not implemented and raises NotImplementedError.

Location

src/database/studies.py:171

def attach_runs(
    study_id: int,
    run_ids: list[int],
    user: User,
    connection: Connection,
) -> None:
    raise NotImplementedError

Expected Behavior

The function should attach multiple runs to a study, similar to how attach_run and attach_tasks work.

Impact

This prevents users from attaching runs to studies programmatically in batch operations.

Suggested Implementation

Follow the pattern used in attach_tasks function (lines 143-164) which validates study ownership and inserts multiple records.

[PGijsbers]

Contained within #55