ci: add GitHub Actions auto-build + fix Unraid update

- Add .github/workflows/docker-publish.yml: builds Docker image on every
  push to main and publishes to ghcr.io/r0m1k3/podcastic:latest automatically

- docker-compose.yml: replace local `build:` with `image: ghcr.io/r0m1k3/podcastic:latest`
  so Unraid's "Update" button can pull the image directly from the registry
  instead of failing with "no service selected"

- Remove deprecated `version: '3.8'` field

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -1,5 +1,3 @@
-version: '3.8'
-
 services:
   mongodb:
     image: mongo:8
@@ -31,9 +29,7 @@ services:
       retries: 5
 
   app:
-    build:
-      context: .
-      dockerfile: Dockerfile
+    image: ghcr.io/r0m1k3/podcastic:latest
     container_name: podcastic-app
     restart: unless-stopped
     ports: