feat: add in-flight RPC tracking, plugin shutdown hooks, and graceful drain

Replace the fixed 30s sleep-based shutdown drain with actual in-flight RPC
tracking using an Arc<AtomicUsize> counter and RAII InFlightGuard. On
SIGTERM/SIGINT the server now:

1. Stops accepting new client and federation connections
2. Sends QUIC CONNECTION_CLOSE with reason "server shutting down"
3. Polls the in-flight counter until it reaches 0 (or drain timeout)
4. Logs drain progress as RPCs complete
5. Calls plugin on_shutdown hooks before exit

Also adds:
- on_shutdown hook to HookVTable (C-ABI plugin API) and ServerHooks trait
- server_in_flight_rpcs Prometheus gauge metric
- Federation connection tracking via shared in-flight counter

crates/quicprochat-server/src/metrics.rs

@@ -56,6 +56,13 @@ pub fn record_storage_latency(operation: &'static str, duration: std::time::Dura
         .record(duration.as_secs_f64());
 }
 
+// ── In-flight RPCs ────────────────────────────────────────────────────────
+
+/// Record the current number of in-flight RPCs (connections being served).
+pub fn record_in_flight_rpcs(count: usize) {
+    metrics::gauge!("server_in_flight_rpcs").set(count as f64);
+}
+
 // ── Server info ────────────────────────────────────────────────────────────
 
 /// Record the server uptime in seconds (set periodically).