feat(TASK-12): periodic telemetry reporter — 15min interval, collector POST

Background task spawned on server startup: every 15 minutes, checks opt-in
status, builds anonymous health report (node ID hash, version, uptime,
CPU/RAM/disk %, container states, recent alerts), saves to disk, and POSTs
to TELEMETRY_COLLECTOR_URL env var if configured. Non-fatal on failure.

Fixed FiredAlert field references (kind not rule_type, timestamp not
fired_at) in both monitoring and analytics modules.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/server.rs

@@ -103,6 +103,7 @@ impl Server {
 
         // Create metrics store and spawn background collector
         let metrics_store = Arc::new(MetricsStore::with_data_dir(config.data_dir.clone()));
+        let metrics_for_telemetry = metrics_store.clone();
         crate::monitoring::spawn_metrics_collector(metrics_store.clone(), Some(state_manager.clone()), Some(config.data_dir.clone()));
 
         let api_handler = Arc::new(
@@ -292,6 +293,13 @@ impl Server {
         // Respects webhook config: skips when disabled or ContainerCrash not subscribed
         crate::health_monitor::spawn_health_monitor(state_manager.clone(), config.data_dir.clone());
 
+        // Periodic telemetry reporter (every 15 min when opted in)
+        crate::monitoring::spawn_telemetry_reporter(
+            metrics_for_telemetry,
+            Some(state_manager.clone()),
+            config.data_dir.clone(),
+        );
+
         Ok(Self {
             _config: config,
             _identity: identity,