feat: wire up federation message routing and P2P client fallback

- Enqueue handler checks resolve_destination() for remote recipients
- User resolution supports user@domain federated addresses
- P2P mesh commands (/mesh start, /mesh stop) wired into client session
- Federation routing integration tests with SqlStore
- Fix DashMap deadlock in validate_session()

crates/quicprochat-server/src/config.rs

@@ -105,7 +105,6 @@ pub struct FederationPeerConfig {
 }
 
 #[derive(Debug)]
-#[allow(dead_code)] // federation not yet wired up
 pub struct EffectiveFederationConfig {
     pub enabled: bool,
     pub domain: String,