Add Gitea CI to build and push images to docker.sejong.dev.

Switch compose to pull sejong-office:latest and document registry secrets and deploy steps.

Co-authored-by: Cursor <cursoragent@cursor.com>

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   backend:
-    build: .
+    image: docker.sejong.dev/sejong-office:latest
     ports:
       - '3000:3000'
     environment: