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/service/webhook/webhook_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/fx"
@@ -17,6 +18,7 @@ import (
 	"sneak.berlin/go/upaas/internal/docker"
 	"sneak.berlin/go/upaas/internal/globals"
 	"sneak.berlin/go/upaas/internal/logger"
+	"sneak.berlin/go/upaas/internal/metrics"
 	"sneak.berlin/go/upaas/internal/models"
 	"sneak.berlin/go/upaas/internal/service/deploy"
 	"sneak.berlin/go/upaas/internal/service/notify"
@@ -63,13 +65,17 @@ func setupTestService(t *testing.T) (*webhook.Service, *database.Database, func(
 	notifySvc, err := notify.New(fx.Lifecycle(nil), notify.ServiceParams{Logger: deps.logger})
 	require.NoError(t, err)
 
+	metricsInstance := metrics.NewForTest(prometheus.NewRegistry())
+
 	deploySvc, err := deploy.New(fx.Lifecycle(nil), deploy.ServiceParams{
 		Logger: deps.logger, Config: deps.config, Database: deps.db, Docker: dockerClient, Notify: notifySvc,
+		Metrics: metricsInstance,
 	})
 	require.NoError(t, err)
 
 	svc, err := webhook.New(fx.Lifecycle(nil), webhook.ServiceParams{
 		Logger: deps.logger, Database: deps.db, Deploy: deploySvc,
+		Metrics: metricsInstance,
 	})
 	require.NoError(t, err)