fix: add periodic flush and graceful shutdown for server-side telemetry

The TelemetryClient only flushed at 50 events, so the server silently
lost all queued telemetry on restart. Add startPeriodicFlush/stop methods
to TelemetryClient, wire up 60s periodic flush in server initTelemetry,
and flush on SIGTERM/SIGINT before exit.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

server/src/telemetry.ts

@@ -21,6 +21,7 @@ export function initTelemetry(fileConfig?: { enabled?: boolean }): TelemetryClie
     () => loadOrCreateState(stateDir, serverVersion),
     serverVersion,
   );
+  client.startPeriodicFlush(60_000);
   return client;
 }