feat: add Helm chart and switch image builds to DOCR

- Add chart/ with backend, frontend, valkey deployments, services, and ingress
- Update nginx.conf to use BACKEND_URL env var via envsubst
- Update Dockerfile.frontend to use nginx template mechanism
- Build.yml: switch Docker registry from Gitea to DOCR, add helm package+push step

nginx.conf

@@ -3,9 +3,9 @@ server {
     root /usr/share/nginx/html;
     index index.html;
 
-    # Proxy API and WebDAV to backend
+    # Proxy API and WebDAV to backend — BACKEND_URL injected at runtime via envsubst
     location /v1/ {
-        proxy_pass http://localhost:8000;
+        proxy_pass ${BACKEND_URL};
         proxy_http_version 1.1;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
@@ -14,7 +14,7 @@ server {
     }
 
     location /webdav/ {
-        proxy_pass http://localhost:8000;
+        proxy_pass ${BACKEND_URL};
         proxy_http_version 1.1;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;