ci(deps): keep actions, docker base and pip deps updated automatically

Dependabot opens weekly PRs for the three update surfaces: github-actions
(grouped into one PR; the first one also clears the Node 20 deprecation
warning on checkout@v4 and upload-artifact@v4), the python base image, and
the pip dependencies. A companion workflow enables auto-merge for minor and
patch bumps so they land on their own once the required make test check is
green; major bumps stay manual.

Requires two repo settings: allow auto-merge, and a branch protection rule
on main with make test as required status check, which is the gate that
keeps auto-merge from landing untested changes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/dependabot.yml

@@ -0,0 +1,21 @@
+---
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    groups:
+      actions:
+        patterns:
+          - "*"
+
+  - package-ecosystem: docker
+    directory: /
+    schedule:
+      interval: weekly
+
+  - package-ecosystem: pip
+    directory: /
+    schedule:
+      interval: weekly

.github/workflows/dependabot-auto-merge.yml

@@ -0,0 +1,29 @@
+---
+name: Dependabot auto-merge
+
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  auto-merge:
+    # Enable auto-merge for minor/patch dependency bumps; majors stay manual.
+    # The merge only happens once every required status check (make test) is
+    # green, so the CI stays the gate.
+    if: github.actor == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Fetch dependency metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v2
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Enable auto-merge (minor + patch)
+        if: steps.metadata.outputs.update-type != 'version-update:semver-major'
+        run: gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}