feat(observability): phase 2 - stale container detection

Add periodic scanner for stale containers:
- Cron-based scanner (hourly) detects non-running containers exceeding threshold
- last_alive_at tracking on instances, updated on deploy/start/restart
- API: GET /api/containers/stale, POST cleanup (single + bulk)
- Event log warnings emitted for newly stale containers
- Graceful handling of externally removed containers

internal/store/models.go

@@ -71,6 +71,7 @@ type Instance struct {
 	NpmProxyID  int    `json:"npm_proxy_id"`
 	Status      string `json:"status"` // running, stopped, failed, removing
 	Port        int    `json:"port"`
+	LastAliveAt string `json:"last_alive_at"`
 	CreatedAt   string `json:"created_at"`
 	UpdatedAt   string `json:"updated_at"`
 }