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/docker-compose.yml

@@ -3,11 +3,11 @@
 # token you enter when adding this host in the central StackPilot UI).
 services:
   agent:
-    image: 10.10.6.10:3020/menzelj/stackpilot-agent:latest
+    image: git.menzel.center/menzeljonas/stackpilot-agent:latest
     build:
       context: .
       args:
-        BACKEND_IMAGE: 10.10.6.10:3020/menzelj/stackpilot-backend:latest
+        BACKEND_IMAGE: git.menzel.center/menzeljonas/stackpilot-backend:latest
     restart: unless-stopped
     environment:
       - AGENT_TOKEN=${AGENT_TOKEN:?set AGENT_TOKEN in .env}