feat(system): process heartbeat + /system/health, engine runs business fields (W9-I)

- process_heartbeat / process_heartbeat_sample (0034): api / worker / worker:agent / all
  upsert every 15s (event loop lag last/max, concurrency, running runs, SSE connections, memory)
- GET /system/health (admin): processes with 45s stale flag, SSE, DB latency/size/connections,
  disks (DATA_DIR / library roots / download dirs, cache thresholds), mount sentinel checks
  with IO timeout, engine pending/oldest/24h failure rate, schedules
- GET /system/health/history?hours=: bucketed lag / running / SSE series
- /engine/runs and /engine/runs/{id}: business {kind, refId, title, triggeredBy} joined from
  run input / task / agent_run / tasks_run_meta / outbox; businessKind / businessRefId filters
- docs/runbook/ops.md: field meanings and alert thresholds

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/server/src/modules/index.ts

@@ -6,6 +6,7 @@ import { piModule } from "./pi/index.ts";
 import { schedulesModule } from "./schedules/index.ts";
 import { searchModule } from "./search/index.ts";
 import { sourcesIdentityModule } from "./sources-identity/index.ts";
+import { systemModule } from "./system/index.ts";
 import type { ServerModule } from "./types.ts";
 
 export type * from "./types.ts";
@@ -23,4 +24,5 @@ export const SERVER_MODULES: readonly ServerModule[] = [
   piModule,
   schedulesModule,
   searchModule,
+  systemModule,
 ];