Add nightly CI base image and migrate workflows to python-repositories-ci.

Pre-build Python, uv, and locked deps in a Gitea container image so daily jobs skip repeated bootstrap; document runner registry auth and ci-bot credentials.

Co-authored-by: Cursor <[email protected]>

.gitea/workflows/dependency-update-bot.yml

@@ -7,22 +7,17 @@ on:
 
 jobs:
   update-check:
-    runs-on: ubuntu-latest
+    runs-on: python-repositories-ci
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
         with:
           token: ${{ secrets.CI_RUNNER_TOKEN }}
 
-      - name: Set up Python
-        uses: actions/setup-python@v5
-        with:
-          python-version-file: .python-version
-
-      - name: Install uv
-        run: pip install uv
-
       - name: Upgrade dependencies
+        env:
+          UV_INDEX_GITEA_USERNAME: ci-bot
+          UV_INDEX_GITEA_PASSWORD: ${{ secrets.CI_RUNNER_TOKEN }}
         run: uv lock --upgrade
 
       - name: Commit and push changes