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/auth.go

@@ -3,6 +3,7 @@ package handlers
 import (
 	"net/http"
 
+	"sneak.berlin/go/upaas/internal/models"
 	"sneak.berlin/go/upaas/templates"
 )
 
@@ -61,6 +62,9 @@ func (h *Handlers) HandleLoginPOST() http.HandlerFunc {
 			return
 		}
 
+		h.auditLog(request, models.AuditActionLogin,
+			models.AuditResourceSession, "", "user logged in")
+
 		http.Redirect(writer, request, "/", http.StatusSeeOther)
 	}
 }
@@ -68,6 +72,9 @@ func (h *Handlers) HandleLoginPOST() http.HandlerFunc {
 // HandleLogout handles logout requests.
 func (h *Handlers) HandleLogout() http.HandlerFunc {
 	return func(writer http.ResponseWriter, request *http.Request) {
+		h.auditLog(request, models.AuditActionLogout,
+			models.AuditResourceSession, "", "user logged out")
+
 		destroyErr := h.auth.DestroySession(writer, request)
 		if destroyErr != nil {
 			h.log.Error("failed to destroy session", "error", destroyErr)