feat: build and publish release images from version tags

Tagging v* builds both images on Gitea Actions and pushes them to the instance
registry, then smoke-tests the published stack. The frontend read its backend
URL through import.meta.env, which Vite resolves at build time, so an image
could only point at whatever the build host had; it now reads it at runtime.

docker-compose.yml

@@ -24,6 +24,10 @@ services:
 
 
   backend:
+    image: git.jaroszew.ski/patrick/chitai-backend:${CHITAI_VERSION:-latest}
+
+    # Only used when the image above is not present locally. Run `docker compose build` to
+    # build from source instead of pulling a release.
     build: ./backend
 
     networks:
@@ -53,6 +57,8 @@ services:
         condition: service_healthy
 
   frontend:
+    image: git.jaroszew.ski/patrick/chitai-frontend:${CHITAI_VERSION:-latest}
+
     build: ./frontend
 
     networks:
@@ -66,6 +72,9 @@ services:
     environment:
       VITE_BACKEND_API_URL: ${CHITAI_API_URL}
 
+      # Must match the URL the browser uses, or SvelteKit rejects form POSTs as cross-origin.
+      ORIGIN: ${CHITAI_ORIGIN:-http://localhost:3000}
+
     depends_on:
       backend:
         condition: service_healthy