ci: add Gitea Actions workflow to build and push Docker image

On push to main (or version tag), the workflow logs into the Gitea container
registry, builds a multi-tag image (sha-<short>, branch name, 'latest' on main,
version on tag) and pushes to gitea.siegeln.net/<owner>/<repo>.

docker-compose.prod.yml now pulls from the registry by default, with
KOCHWAS_TAG env var to pin a specific build.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.prod.yml

@@ -1,7 +1,9 @@
 services:
   kochwas:
-    build: .
-    image: kochwas:latest
+    # Image wird via Gitea Actions gebaut und in die Gitea-Container-Registry gepusht.
+    # Tag-Override per Umgebungsvariable: KOCHWAS_TAG=sha-abcd1234 docker compose pull
+    image: gitea.siegeln.net/claude/kochwas:${KOCHWAS_TAG:-latest}
+    pull_policy: always
     ports:
       - '3000:3000'
     volumes: