feat(relay): replace global Mutex<RoomManager> with DashMap sharding
Eliminates the single-lock bottleneck for media forwarding. Before:
all participants across all rooms competed for one Mutex. Now rooms
are stored in DashMap (64 internal shards with per-shard RwLocks).
Changes:
- RoomManager.rooms: HashMap → DashMap<String, Room>
- Per-room quality tracking (qualities, current_tier moved into Room)
- Arc<Mutex<RoomManager>> → Arc<RoomManager> everywhere
- 20 .lock().await sites removed across room.rs, main.rs, federation.rs, ws.rs
- federation forward_to_peers: clone peer list, release lock, then send
- ACL uses std::sync::Mutex (rarely accessed, non-async)
Concurrency improvement:
- Before: 100 rooms × 10 people = 1000 tasks → 1 Mutex
- After: distributed across 64 DashMap shards, ~15 tasks per shard avg
- Rooms are fully independent — room A never blocks room B
314 tests passing, 0 regressions.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
a52b011fb5