feat: add observability improvements (metrics, audit log, structured logging)

- Add Prometheus metrics package (internal/metrics) with deployment,
  container health, webhook, HTTP request, and audit counters/histograms
- Add audit_log SQLite table via migration 007
- Add AuditEntry model with CRUD operations and query methods
- Add audit service (internal/service/audit) for recording user actions
- Instrument deploy service with deployment duration, count, and
  in-flight metrics; container health gauge updates on deploy completion
- Instrument webhook service with event counters by app/type/matched
- Instrument HTTP middleware with request count, duration, and response
  size metrics; also log response bytes in structured request logs
- Add audit logging to all key handler operations: login/logout, app
  CRUD, deploy, cancel, rollback, restart/stop/start, webhook receipt,
  and initial setup
- Add GET /api/audit endpoint for querying recent audit entries
- Make /metrics endpoint always available (optionally auth-protected)
- Add comprehensive tests for metrics, audit model, and audit service
- Update existing test infrastructure with metrics and audit dependencies
- Update README with Observability section documenting all metrics,
  audit log, and structured logging

internal/metrics/metrics.go

@@ -0,0 +1,148 @@
+// Package metrics provides Prometheus metrics for upaas.
+//
+//nolint:revive // "metrics" matches the domain; runtime/metrics is rarely imported directly
+package metrics
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+	"go.uber.org/fx"
+)
+
+// Params contains dependencies for Metrics.
+type Params struct {
+	fx.In
+}
+
+// Metrics holds all Prometheus metrics for the application.
+type Metrics struct {
+	// Deployment metrics.
+	DeploymentsTotal    *prometheus.CounterVec
+	DeploymentDuration  *prometheus.HistogramVec
+	DeploymentsInFlight *prometheus.GaugeVec
+
+	// Container health metrics.
+	ContainerHealthy *prometheus.GaugeVec
+
+	// Webhook metrics.
+	WebhookEventsTotal *prometheus.CounterVec
+
+	// HTTP request metrics.
+	HTTPRequestsTotal     *prometheus.CounterVec
+	HTTPRequestDuration   *prometheus.HistogramVec
+	HTTPResponseSizeBytes *prometheus.HistogramVec
+
+	// Audit log metrics.
+	AuditEventsTotal *prometheus.CounterVec
+}
+
+// New creates a new Metrics instance with all Prometheus metrics registered
+// in the default Prometheus registry.
+func New(_ fx.Lifecycle, _ Params) (*Metrics, error) {
+	return newMetrics(promauto.With(prometheus.DefaultRegisterer)), nil
+}
+
+// NewForTest creates a Metrics instance with a custom registry for test isolation.
+func NewForTest(reg prometheus.Registerer) *Metrics {
+	return newMetrics(promauto.With(reg))
+}
+
+// newMetrics creates a Metrics instance using the given factory.
+func newMetrics(factory promauto.Factory) *Metrics {
+	return &Metrics{
+		DeploymentsTotal:      newDeploymentsTotal(factory),
+		DeploymentDuration:    newDeploymentDuration(factory),
+		DeploymentsInFlight:   newDeploymentsInFlight(factory),
+		ContainerHealthy:      newContainerHealthy(factory),
+		WebhookEventsTotal:    newWebhookEventsTotal(factory),
+		HTTPRequestsTotal:     newHTTPRequestsTotal(factory),
+		HTTPRequestDuration:   newHTTPRequestDuration(factory),
+		HTTPResponseSizeBytes: newHTTPResponseSizeBytes(factory),
+		AuditEventsTotal:      newAuditEventsTotal(factory),
+	}
+}
+
+func newDeploymentsTotal(f promauto.Factory) *prometheus.CounterVec {
+	return f.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "upaas",
+		Subsystem: "deployments",
+		Name:      "total",
+		Help:      "Total number of deployments by app and status.",
+	}, []string{"app", "status"})
+}
+
+func newDeploymentDuration(f promauto.Factory) *prometheus.HistogramVec {
+	return f.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: "upaas",
+		Subsystem: "deployments",
+		Name:      "duration_seconds",
+		Help:      "Duration of deployments in seconds by app and status.",
+		Buckets:   []float64{10, 30, 60, 120, 300, 600, 1800},
+	}, []string{"app", "status"})
+}
+
+func newDeploymentsInFlight(f promauto.Factory) *prometheus.GaugeVec {
+	return f.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "upaas",
+		Subsystem: "deployments",
+		Name:      "in_flight",
+		Help:      "Number of deployments currently in progress by app.",
+	}, []string{"app"})
+}
+
+func newContainerHealthy(f promauto.Factory) *prometheus.GaugeVec {
+	return f.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "upaas",
+		Subsystem: "container",
+		Name:      "healthy",
+		Help:      "Whether the app container is healthy (1) or unhealthy (0).",
+	}, []string{"app"})
+}
+
+func newWebhookEventsTotal(f promauto.Factory) *prometheus.CounterVec {
+	return f.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "upaas",
+		Subsystem: "webhook",
+		Name:      "events_total",
+		Help:      "Total number of webhook events by app, event type, and matched status.",
+	}, []string{"app", "event_type", "matched"})
+}
+
+func newHTTPRequestsTotal(f promauto.Factory) *prometheus.CounterVec {
+	return f.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "upaas",
+		Subsystem: "http",
+		Name:      "requests_total",
+		Help:      "Total number of HTTP requests by method and status code.",
+	}, []string{"method", "status_code"})
+}
+
+func newHTTPRequestDuration(f promauto.Factory) *prometheus.HistogramVec {
+	return f.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: "upaas",
+		Subsystem: "http",
+		Name:      "request_duration_seconds",
+		Help:      "Duration of HTTP requests in seconds by method.",
+		Buckets:   prometheus.DefBuckets,
+	}, []string{"method"})
+}
+
+//nolint:mnd // bucket boundaries are domain-specific constants
+func newHTTPResponseSizeBytes(f promauto.Factory) *prometheus.HistogramVec {
+	return f.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: "upaas",
+		Subsystem: "http",
+		Name:      "response_size_bytes",
+		Help:      "Size of HTTP responses in bytes by method.",
+		Buckets:   prometheus.ExponentialBuckets(100, 10, 7),
+	}, []string{"method"})
+}
+
+func newAuditEventsTotal(f promauto.Factory) *prometheus.CounterVec {
+	return f.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "upaas",
+		Subsystem: "audit",
+		Name:      "events_total",
+		Help:      "Total number of audit log events by action.",
+	}, []string{"action"})
+}