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_test.go

@@ -0,0 +1,158 @@
+package metrics_test
+
+import (
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/fx"
+
+	"sneak.berlin/go/upaas/internal/metrics"
+)
+
+func TestNewForTest(t *testing.T) {
+	t.Parallel()
+
+	reg := prometheus.NewRegistry()
+	m := metrics.NewForTest(reg)
+
+	require.NotNil(t, m)
+	assert.NotNil(t, m.DeploymentsTotal)
+	assert.NotNil(t, m.DeploymentDuration)
+	assert.NotNil(t, m.DeploymentsInFlight)
+	assert.NotNil(t, m.ContainerHealthy)
+	assert.NotNil(t, m.WebhookEventsTotal)
+	assert.NotNil(t, m.HTTPRequestsTotal)
+	assert.NotNil(t, m.HTTPRequestDuration)
+	assert.NotNil(t, m.HTTPResponseSizeBytes)
+	assert.NotNil(t, m.AuditEventsTotal)
+}
+
+func TestNew(t *testing.T) {
+	t.Parallel()
+
+	m, err := metrics.New(fx.Lifecycle(nil), metrics.Params{})
+	require.NoError(t, err)
+	require.NotNil(t, m)
+}
+
+func TestDeploymentMetrics(t *testing.T) {
+	t.Parallel()
+
+	reg := prometheus.NewRegistry()
+	m := metrics.NewForTest(reg)
+
+	m.DeploymentsTotal.WithLabelValues("test-app", "success").Inc()
+	m.DeploymentDuration.WithLabelValues("test-app", "success").Observe(42.5)
+	m.DeploymentsInFlight.WithLabelValues("test-app").Set(1)
+
+	families, err := reg.Gather()
+	require.NoError(t, err)
+
+	names := make(map[string]bool)
+
+	for _, f := range families {
+		names[f.GetName()] = true
+	}
+
+	assert.True(t, names["upaas_deployments_total"])
+	assert.True(t, names["upaas_deployments_duration_seconds"])
+	assert.True(t, names["upaas_deployments_in_flight"])
+}
+
+func TestContainerHealthMetrics(t *testing.T) {
+	t.Parallel()
+
+	reg := prometheus.NewRegistry()
+	m := metrics.NewForTest(reg)
+
+	m.ContainerHealthy.WithLabelValues("my-app").Set(1)
+
+	families, err := reg.Gather()
+	require.NoError(t, err)
+
+	found := false
+
+	for _, f := range families {
+		if f.GetName() == "upaas_container_healthy" {
+			found = true
+
+			break
+		}
+	}
+
+	assert.True(t, found)
+}
+
+func TestWebhookMetrics(t *testing.T) {
+	t.Parallel()
+
+	reg := prometheus.NewRegistry()
+	m := metrics.NewForTest(reg)
+
+	m.WebhookEventsTotal.WithLabelValues("test-app", "push", "true").Inc()
+
+	families, err := reg.Gather()
+	require.NoError(t, err)
+
+	found := false
+
+	for _, f := range families {
+		if f.GetName() == "upaas_webhook_events_total" {
+			found = true
+
+			break
+		}
+	}
+
+	assert.True(t, found)
+}
+
+func TestHTTPMetrics(t *testing.T) {
+	t.Parallel()
+
+	reg := prometheus.NewRegistry()
+	m := metrics.NewForTest(reg)
+
+	m.HTTPRequestsTotal.WithLabelValues("GET", "200").Inc()
+	m.HTTPRequestDuration.WithLabelValues("GET").Observe(0.05)
+	m.HTTPResponseSizeBytes.WithLabelValues("GET").Observe(1024)
+
+	families, err := reg.Gather()
+	require.NoError(t, err)
+
+	names := make(map[string]bool)
+
+	for _, f := range families {
+		names[f.GetName()] = true
+	}
+
+	assert.True(t, names["upaas_http_requests_total"])
+	assert.True(t, names["upaas_http_request_duration_seconds"])
+	assert.True(t, names["upaas_http_response_size_bytes"])
+}
+
+func TestAuditMetrics(t *testing.T) {
+	t.Parallel()
+
+	reg := prometheus.NewRegistry()
+	m := metrics.NewForTest(reg)
+
+	m.AuditEventsTotal.WithLabelValues("login").Inc()
+
+	families, err := reg.Gather()
+	require.NoError(t, err)
+
+	found := false
+
+	for _, f := range families {
+		if f.GetName() == "upaas_audit_events_total" {
+			found = true
+
+			break
+		}
+	}
+
+	assert.True(t, found)
+}