fix(ingress): migrate from nginx to Traefik ingress controller

- Change ingressClassName from nginx to traefik
- Replace nginx configuration-snippet annotations with Traefik router annotations
- Extract security headers into dedicated Traefik Middleware CRD
- Update CI/CD pipeline to apply middleware manifest
- Document migration decision and deployment steps in README

ADR-002: Ingress Controller Migration (nginx → Traefik)
Migration strategy: in-place update

k8s/middleware.yaml

@@ -0,0 +1,22 @@
+apiVersion: traefik.io/v1alpha1
+kind: Middleware
+metadata:
+  name: security-headers
+  namespace: openclaw-private
+  labels:
+    app.kubernetes.io/name: signalledger
+    app.kubernetes.io/component: middleware
+    app.kubernetes.io/part-of: signalledger
+spec:
+  headers:
+    customRequestHeaders:
+      X-Forwarded-Proto: "https"
+    customResponseHeaders:
+      Strict-Transport-Security: "max-age=31536000; includeSubDomains; preload"
+      X-Frame-Options: "DENY"
+      X-Content-Type-Options: "nosniff"
+      Referrer-Policy: "strict-origin-when-cross-origin"
+      Content-Security-Policy: "default-src 'self'; script-src 'self' 'unsafe-inline' https://pagead2.googlesyndication.com https://partner.googleadservices.com https://tpc.googlesyndication.com; img-src 'self' data: https:; style-src 'self' 'unsafe-inline'; font-src 'self'; connect-src 'self'; frame-src https://googleads.g.doubleclick.net; object-src 'none'; base-uri 'self'; form-action 'self';"
+      X-XSS-Protection: "1; mode=block"
+      Permissions-Policy: "accelerometer=(), camera=(), geolocation=(), gyroscope=(), magnetometer=(), microphone=(), payment=(), usb=()"
+    sslRedirect: true