P6-04+05: Prometheus /metrics endpoint + Grafana dashboard

New internal/server/metrics package emits the legacy text/plain
exposition format directly, so we don't pull in
prometheus/client_golang. Endpoint is opt-in via RM_METRICS_TOKEN
and/or RM_METRICS_TRUSTED_CIDR; route is not mounted at all if
neither gate is set. Both gates ANDed when both configured.

Per-host gauges (online, last_backup_*, repo_size_bytes,
snapshot_count, open_alerts, repo_status), server gauges
(hosts_total/online, active_alerts by severity, build_info), and
an in-memory job-duration histogram observed from the existing
MsgJobFinished branch in the WS handler.

Docs in docs/prometheus.md (enable + scrape config + metric
reference + dashboard import). Sample dashboard at
deploy/grafana/restic-manager-dashboard.json - six panels,
Grafana schema 39, single Prometheus datasource variable.

Tests: golden render, concurrent observe, bucket boundaries in
the metrics package; auth matrix (no auth -> 404, token gate,
CIDR gate, both required) in the HTTP layer.

internal/server/http/server.go

@@ -17,6 +17,7 @@ import (
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/crypto"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/notification"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/config"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/metrics"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/oidc"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/ui"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/ws"
@@ -56,6 +57,12 @@ type Deps struct {
 	// OIDC (optional). Non-nil when the operator has configured an
 	// IdP — handlers under /auth/oidc/* are mounted only when set.
 	OIDC *oidc.Client
+	// Metrics (optional). When non-nil the WS job-finished branch
+	// records job durations and the /metrics handler can pull a
+	// histogram snapshot. Independent of MetricsAuthEnabled — the
+	// recorder runs even if the scrape endpoint is gated off, so a
+	// later config flip doesn't lose the running window.
+	Metrics *metrics.Registry
 }
 
 // Server is the running HTTP server.
@@ -131,12 +138,16 @@ func (s *Server) routes(r chi.Router) {
 	r.Get("/agent/binary", s.handleAgentBinary)
 	r.Get("/install/*", s.handleInstallAsset)
 	r.Get("/api/version", s.handleVersion)
+	if s.deps.Cfg.MetricsAuthEnabled() {
+		r.Get("/metrics", s.handleMetrics)
+	}
 	if s.deps.Hub != nil {
 		hd := ws.HandlerDeps{
 			Hub:            s.deps.Hub,
 			Store:          s.deps.Store,
 			JobHub:         s.deps.JobHub,
 			AlertEngine:    s.deps.AlertEngine,
+			Metrics:        s.deps.Metrics,
 			OnHello:        s.onAgentHello,
 			OnScheduleAck:  s.applyScheduleAck,
 			OnScheduleFire: s.dispatchScheduledJob,