Implement hybrid database + application-level enforcement to ensure users
can only be in one room at a time, with graceful auto-leave behavior and
clear error messaging.
## Changes
### Database Layer
- Add unique index on `room_members.user_id` to enforce one room per user
- Migration includes cleanup of any existing duplicate memberships
- Constraint provides safety net if application logic fails
### Application Layer
- Auto-leave logic: when joining a room, automatically remove user from
all other rooms first
- Return `AutoLeaveResult` with metadata about rooms that were left
- Idempotent rejoining: rejoining the same room just updates status
### API Layer
- Join route returns auto-leave information in response
- Catches and handles constraint violations with 409 Conflict
- User-friendly error messages when conflicts occur
### Frontend
- Room list and detail pages handle ROOM_MEMBERSHIP_CONFLICT errors
- Show alerts when user needs to leave current room
- Refresh room list after conflicts to show current state
### Testing
- 7 integration tests for modal room behavior
- Tests cover: first join, auto-leave, rejoining, multi-user scenarios,
constraint enforcement, and metadata accuracy
- Updated existing unit tests for new return signature
## Technical Details
- `addRoomMember()` now returns `{ member, autoLeaveResult? }`
- Auto-leave happens before new room join, preventing race conditions
- Database unique constraint as ultimate safety net
- Socket events remain status-only (joining goes through API)
## Testing
- ✅ All modal room tests pass (7/7)
- ✅ All room API e2e tests pass (12/12)
- ✅ Format and lint checks pass
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
Thomas Hallock