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/handlers/handlers_test.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/fx"
@@ -24,8 +25,10 @@ import (
 	"sneak.berlin/go/upaas/internal/handlers"
 	"sneak.berlin/go/upaas/internal/healthcheck"
 	"sneak.berlin/go/upaas/internal/logger"
+	"sneak.berlin/go/upaas/internal/metrics"
 	"sneak.berlin/go/upaas/internal/middleware"
 	"sneak.berlin/go/upaas/internal/service/app"
+	"sneak.berlin/go/upaas/internal/service/audit"
 	"sneak.berlin/go/upaas/internal/service/auth"
 	"sneak.berlin/go/upaas/internal/service/deploy"
 	"sneak.berlin/go/upaas/internal/service/notify"
@@ -92,7 +95,8 @@ func createAppServices(
 	logInstance *logger.Logger,
 	dbInstance *database.Database,
 	cfg *config.Config,
-) (*auth.Service, *app.Service, *deploy.Service, *webhook.Service, *docker.Client) {
+	metricsInstance *metrics.Metrics,
+) (*auth.Service, *app.Service, *deploy.Service, *webhook.Service, *docker.Client, *audit.Service) {
 	t.Helper()
 
 	authSvc, authErr := auth.New(fx.Lifecycle(nil), auth.ServiceParams{
@@ -125,6 +129,7 @@ func createAppServices(
 		Database: dbInstance,
 		Docker:   dockerClient,
 		Notify:   notifySvc,
+		Metrics:  metricsInstance,
 	})
 	require.NoError(t, deployErr)
 
@@ -132,10 +137,18 @@ func createAppServices(
 		Logger:   logInstance,
 		Database: dbInstance,
 		Deploy:   deploySvc,
+		Metrics:  metricsInstance,
 	})
 	require.NoError(t, webhookErr)
 
-	return authSvc, appSvc, deploySvc, webhookSvc, dockerClient
+	auditSvc, auditErr := audit.New(fx.Lifecycle(nil), audit.ServiceParams{
+		Logger:   logInstance,
+		Database: dbInstance,
+		Metrics:  metricsInstance,
+	})
+	require.NoError(t, auditErr)
+
+	return authSvc, appSvc, deploySvc, webhookSvc, dockerClient, auditSvc
 }
 
 func setupTestHandlers(t *testing.T) *testContext {
@@ -145,11 +158,14 @@ func setupTestHandlers(t *testing.T) *testContext {
 
 	globalInstance, logInstance, dbInstance, hcInstance := createCoreServices(t, cfg)
 
-	authSvc, appSvc, deploySvc, webhookSvc, dockerClient := createAppServices(
+	metricsInstance := metrics.NewForTest(prometheus.NewRegistry())
+
+	authSvc, appSvc, deploySvc, webhookSvc, dockerClient, auditSvc := createAppServices(
 		t,
 		logInstance,
 		dbInstance,
 		cfg,
+		metricsInstance,
 	)
 
 	handlersInstance, handlerErr := handlers.New(
@@ -164,6 +180,7 @@ func setupTestHandlers(t *testing.T) *testContext {
 			Deploy:      deploySvc,
 			Webhook:     webhookSvc,
 			Docker:      dockerClient,
+			Audit:       auditSvc,
 		},
 	)
 	require.NoError(t, handlerErr)
@@ -173,6 +190,7 @@ func setupTestHandlers(t *testing.T) *testContext {
 		Globals: globalInstance,
 		Config:  cfg,
 		Auth:    authSvc,
+		Metrics: metricsInstance,
 	})
 	require.NoError(t, mwErr)