- add backend media-group endpoint with atomic message and attachment creation
- switch web composer to send multi-media albums through the new endpoint
- switch Android batch media sending to upload first and create one album message with caption
- add reply_to/forwarded_from message fields and chat pinned_message field
- add forward and pin APIs plus reply support in message create
- wire web actions: Reply, Fwd, Pin and reply composer state
- fix spam policy bug: allow repeated identical messages, keep rate limiting