security: upgrade Gitea to 1.25.4, add security headers

- Update Gitea from 1.22.6 to 1.25.4 (fixes CVE-2026-20736, CVE-2026-20912)
- Disable public registration
- Disable Swagger API
- Add nginx-ingress security headers:
  - X-Content-Type-Options: nosniff
  - X-XSS-Protection: 1; mode=block
  - Referrer-Policy: strict-origin-when-cross-origin
  - Permissions-Policy
- Enable HSTS preload
- Reorganize Gitea K8s manifests into gitea/ directory

Made-with: Cursor

backend/deploy/k8s/gitea/namespace.yaml

@@ -0,0 +1,7 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: gitea
+  labels:
+    app.kubernetes.io/name: gitea
+    app.kubernetes.io/part-of: gooseek