Delete File

Member deletes their own file — bucket object(s) + store record.

Works on both pending (cancels the upload) and active files. Ownership scoping is identical to get_file/download_file: wrong member, wrong kind, and unknown file_id are indistinguishable 404s (no existence oracle). Refs to a deleted file already in a conversation's history resolve via the same expired/missing-ref placeholder path as any other reclaimed object — nothing else to update there.