Context

PR #23537 adds a file_ids uuid[] column to chats. File IDs are appended when messages reference uploaded files, but no code path ever removes entries from this array.

Problem

• Editing a message to replace file A with file B leaves A in file_ids (phantom reference).
• Soft-deleted messages' file references persist in the array.
• Over time, chats accumulate stale entries (capped at 20 by MaxChatFileIDs).
• No dbpurge job exists to clean up orphaned chat_files rows.

Suggested approach

1. Add a dbpurge job that periodically scans for chat_files rows not referenced by any chat's file_ids array.
2. Optionally, add a RemoveChatFileIDs query to scrub stale entries when messages are soft-deleted.

The current cap of 20 limits the practical impact, but this should be addressed before the feature leaves experimental.

Ref: #23537 (comment)