Add Hub Messages, Files, Users, Reactions, and Pins APIs
Complete backend for SprintChat Hub migration:
- Messages: send, edit, delete, list (paginated cursor), thread, search
- Files: upload (multipart), download, thumbnail, info, list
- Users: get, getByIds, search, status (online detection)
- Reactions: add, remove, list (grouped by emoji)
- Pins: pin, unpin, list (with message content)
- Channel stats: member/message/pinned/unread counts
4 new DB tables: Hub_Messages, Hub_Files, Hub_Reactions, Hub_PinnedPosts
21 new endpoints added to PUBLIC_ROUTES
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Mike
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