feat(monitor): active push loop replacing standalone monitor

Adds a periodic POST loop to <backend>/monitor/server/heartbeat so
HF plugin can take over the standalone harborforge-monitor daemon's
job — same X-API-Key header, same flat telemetry shape (cpu_pct /
mem_pct / disk_pct / swap_pct / load_avg / uptime_seconds /
plugin_version / agents[]). HF backend stays unchanged.

Config: monitor_push_enabled (default false; opt-in to avoid surprise
heartbeats from existing deployments), monitor_push_interval_seconds
(default 30), reuses apiKey for the X-API-Key header. Lift the
container's HF_MONITER_API_KEY into config.apiKey, flip
monitor_push_enabled true, then docker rm -f the container — DB
last_seen_at keeps advancing under the plugin's loop.

Collector grew swap + cpu sampling (two reads of /proc/stat over a
1-second window when SampleCPU=true). Bridge endpoint stays cheap
(SampleCPU=false on demand); push loop is the only caller paying the
sampling cost.

E2E in sim: monitor_push_enabled=true + apiKey from injected
MonitoredServer row → server_states.last_seen_at advances exactly
every interval_seconds (10s configured, 10s observed). cpu/mem/disk/
swap_pct all populate correctly.

internal/config/config.go

@@ -35,6 +35,17 @@ type Config struct {
 	// server listens on. Zero/missing disables the bridge entirely.
 	MonitorPort int `json:"monitor_port,omitempty"`
 
+	// MonitorPushEnabled toggles the active push loop that uploads
+	// system telemetry to BackendURL /monitor/server/heartbeat. Lets
+	// HF plugin replace the standalone harborforge-monitor container.
+	// nil (unset) defaults to false; operators must opt in explicitly
+	// since they need to provision APIKey too.
+	MonitorPushEnabled *bool `json:"monitor_push_enabled,omitempty"`
+
+	// MonitorPushIntervalSeconds — defaults to 30s when ≤0. Mirrors
+	// the standalone monitor's HF_MONITER_REPORT_INTERVAL knob.
+	MonitorPushIntervalSeconds int `json:"monitor_push_interval_seconds,omitempty"`
+
 	// CalendarHeartbeatIntervalSeconds — defaults to 30s when ≤0.
 	CalendarHeartbeatIntervalSeconds int `json:"calendar_heartbeat_interval_seconds,omitempty"`
 
@@ -57,6 +68,8 @@ type Resolved struct {
 	Identifier                       string
 	APIKey                           string
 	MonitorPort                      int
+	MonitorPushEnabled               bool
+	MonitorPushIntervalSeconds       int
 	CalendarEnabled                  bool
 	CalendarHeartbeatIntervalSeconds int
 	CalendarBackendURL               string
@@ -104,6 +117,8 @@ func Resolve(c Config) Resolved {
 		Identifier:                       c.Identifier,
 		APIKey:                           c.APIKey,
 		MonitorPort:                      c.MonitorPort,
+		MonitorPushEnabled:               false,
+		MonitorPushIntervalSeconds:       30,
 		CalendarEnabled:                  true,
 		CalendarHeartbeatIntervalSeconds: 30,
 		CalendarBackendURL:               c.CalendarBackendURL,
@@ -127,6 +142,12 @@ func Resolve(c Config) Resolved {
 	if c.CalendarHeartbeatIntervalSeconds > 0 {
 		out.CalendarHeartbeatIntervalSeconds = c.CalendarHeartbeatIntervalSeconds
 	}
+	if c.MonitorPushEnabled != nil {
+		out.MonitorPushEnabled = *c.MonitorPushEnabled
+	}
+	if c.MonitorPushIntervalSeconds > 0 {
+		out.MonitorPushIntervalSeconds = c.MonitorPushIntervalSeconds
+	}
 	if c.RestartPollIntervalSeconds > 0 {
 		out.RestartPollIntervalSeconds = c.RestartPollIntervalSeconds
 	}