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>

crates/wzp-relay/Cargo.toml

@@ -28,6 +28,7 @@ prometheus = "0.13"
 axum = { version = "0.7", default-features = false, features = ["tokio", "http1", "ws"] }
 tower-http = { version = "0.6", features = ["fs"] }
 futures-util = "0.3"
+dashmap = "6"
 dirs = "6"
 sha2 = { workspace = true }
 chrono = "0.4"