Adds named group threads (chat_rooms + chat_room_members), mirroring
the collab-buckets membership pattern. Group messages share the
chat_messages table via a new nullable room_uid column; recipient_id
is left 0 for group rows and fan-out happens via the member-join at
GET time instead of duplicating per recipient.
Chat retention flips from "drop everything after 7 days" to
persistent-by-default: a new `persistent` flag on chat_messages marks
long-term rows (default 1 — all group messages plus non-disposable
1:1). The 7-day cleanup only targets persistent=0 rows so the sender
can still poll final receipt state before they're pruned.
New endpoints:
POST/GET/PATCH/DELETE /api/chat/rooms[/{uid}] — room CRUD
GET /api/chat/backfill — scrollback / cold-start history fetch
POST /api/chat — accepts recipient OR room_uid + disposable flag
GET /api/chat — returns 1:1 to caller AND rooms caller is in
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
thandClaude Opus 4.6
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