Fix CI for the user-scoped runners

- ci.yml: connect to the test MariaDB as root so Django creates the test
  database itself (no client install/grant step), and drop actions/cache
  (cache: pip/npm): Gitea's cache service hangs the job on restore/save.
- publish.yml: prefer the REGISTRY_USER/REGISTRY_TOKEN secrets (as on other
  repos) and fall back to the automatic Actions token.
- AGENTS.md: note the CI layout, the runner labels and the cache caveat.

.gitea/workflows/publish.yml

@@ -30,8 +30,10 @@ jobs:
     name: Build & push images
     runs-on: ubuntu-latest
     env:
-      REGISTRY_USER: ${{ github.actor }}
-      REGISTRY_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      # Prefer dedicated registry secrets (as on other repos); fall back to
+      # the automatic Actions token.
+      REGISTRY_USER: ${{ secrets.REGISTRY_USER || github.actor }}
+      REGISTRY_TOKEN: ${{ secrets.REGISTRY_TOKEN || secrets.GITHUB_TOKEN }}
       PLATFORMS: ${{ inputs.platforms || 'linux/amd64,linux/arm64' }}
     steps:
       - uses: actions/checkout@v4