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.go

@@ -7,12 +7,14 @@ import (
 	"encoding/json"
 	"fmt"
 	"log/slog"
+	"strconv"
 
 	"go.uber.org/fx"
 
 	"sneak.berlin/go/upaas/internal/database"
 
 	"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"
 )
@@ -24,23 +26,26 @@ type ServiceParams struct {
 	Logger   *logger.Logger
 	Database *database.Database
 	Deploy   *deploy.Service
+	Metrics  *metrics.Metrics
 }
 
 // Service provides webhook handling functionality.
 type Service struct {
-	log    *slog.Logger
-	db     *database.Database
-	deploy *deploy.Service
-	params *ServiceParams
+	log     *slog.Logger
+	db      *database.Database
+	deploy  *deploy.Service
+	metrics *metrics.Metrics
+	params  *ServiceParams
 }
 
 // New creates a new webhook Service.
 func New(_ fx.Lifecycle, params ServiceParams) (*Service, error) {
 	return &Service{
-		log:    params.Logger.Get(),
-		db:     params.Database,
-		deploy: params.Deploy,
-		params: &params,
+		log:     params.Logger.Get(),
+		db:      params.Database,
+		deploy:  params.Deploy,
+		metrics: params.Metrics,
+		params:  &params,
 	}, nil
 }
 
@@ -122,6 +127,10 @@ func (svc *Service) HandleWebhook(
 		"commit", commitSHA,
 	)
 
+	svc.metrics.WebhookEventsTotal.WithLabelValues(
+		app.Name, eventType, strconv.FormatBool(matched),
+	).Inc()
+
 	// If branch matches, trigger deployment
 	if matched {
 		svc.triggerDeployment(ctx, app, event)