buzz-db: split channel.rs into channel CRUD + membership #7

Problem

channel.rs is 3,419 lines mixing two concerns backed by different database tables:

• Channel CRUD (~1,200 lines, channels table): create_channel, get_channel, update_channel, archive_channel, list_channels, set_topic, set_purpose, etc.
• Membership management (~1,500 lines, channel_members table): add_member, remove_member, is_member, get_members, get_accessible_channel_ids, lock_member_snapshot, get_member_role, advisory-lock helpers, roster fence verification

Plus ~700 lines of tests.

Proposed change

Split into:

• channel.rs — channel CRUD, types (ChannelRecord, ChannelUpdate, AccessibleChannel), and channel-scoped queries
• channel_members.rs — membership operations, types (MemberRecord, LockedMemberSnapshot, BotMemberRecord), advisory-lock helpers, roster fence verification

Priority

Medium — second-largest domain file with a clear natural seam at the table boundary.

🤖 AI review update (2026-08-23)

The seam is valid. Split channel CRUD from membership/roster locking as one self-contained #2 slice, moving the relevant tests and datastore spans in the same PR. Keep TTL/channel-lifecycle locking with the domain that owns its invariant rather than grouping solely by table name.