ci: point images at git.menzel.center and add build-and-push workflow

Image references still pointed at the old server (10.10.6.10:3020/menzelj);
menzelj was never a valid namespace there either, the actual account is
menzeljonas. Also adds .gitea/workflows/ci.yml to build and push
backend, frontend and agent on push to main.

agent/Dockerfile

@@ -1,6 +1,6 @@
 # The agent reuses the backend image (same compose/Docker code + deps) and
 # just runs a different ASGI app. Build the backend image first.
-ARG BACKEND_IMAGE=10.10.6.10:3020/menzelj/stackpilot-backend:latest
+ARG BACKEND_IMAGE=git.menzel.center/menzeljonas/stackpilot-backend:latest
 FROM ${BACKEND_IMAGE}
 
 ENV STACKS_DIR=/opt/stacks \