Extract connection pool management and helper utilities.
Remove redundant metrics indirection, expose counters directly.
Trim room.rs boilerplate and move AI queue logic to room_ai_queue.
- Remove all use slog::* imports and log: slog::Logger fields
- Replace slog macros with tracing::{info!, warn!, error!, debug!}
- metrics.rs: upgrade metrics 0.21→0.22, remove register_*! macros,
use functional API: metrics::gauge!(), metrics::counter!(),
metrics::histogram!(), metrics::describe_gauge!() etc.
- RoomMetrics: all fields now use functional metrics API, dynamic
room_id labels passed as owned String to avoid lifetime issues
- RoomService: remove pub log: slog::Logger field
- connection.rs: remove log from subscribe_room_events,
subscribe_project_room_events, subscribe_task_events_fn
