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>

Cargo.lock

@@ -1191,6 +1191,20 @@ dependencies = [
  "syn 2.0.117",
 ]
 
+[[package]]
+name = "dashmap"
+version = "6.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5041cc499144891f3790297212f32a74fb938e5136a14943f338ef9e0ae276cf"
+dependencies = [
+ "cfg-if",
+ "crossbeam-utils",
+ "hashbrown 0.14.5",
+ "lock_api",
+ "once_cell",
+ "parking_lot_core",
+]
+
 [[package]]
 name = "dasp"
 version = "0.11.0"
@@ -2341,6 +2355,12 @@ version = "0.12.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8a9ee70c43aaf417c914396645a0fa852624801b24ebb7ae78fe8272889ac888"
 
+[[package]]
+name = "hashbrown"
+version = "0.14.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e5274423e17b7c9fc20b6e7e208532f9b19825d82dfd615708b70edd83df41f1"
+
 [[package]]
 name = "hashbrown"
 version = "0.15.5"
@@ -7785,6 +7805,7 @@ dependencies = [
  "axum 0.7.9",
  "bytes",
  "chrono",
+ "dashmap",
  "dirs",
  "futures-util",
  "prometheus",