ci: group Dependabot updates and auto-merge minor and patch bumps

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: cargo
+    directory: /
+    schedule:
+      interval: monthly
+    cooldown:
+      default-days: 7
+    open-pull-requests-limit: 5
+    labels: [dependencies]
+    commit-message:
+      prefix: chore
+      prefix-development: chore
+      include: scope
+    groups:
+      cargo:
+        update-types: [minor, patch]
+      cargo-security:
+        applies-to: security-updates
+        patterns: ["*"]
+    # Majors stay ungrouped: one PR each, never auto-merged.
+
+  - package-ecosystem: docker
+    directory: /
+    schedule:
+      interval: monthly
+    cooldown:
+      default-days: 7
+    labels: [dependencies]
+    commit-message:
+      prefix: chore
+      include: scope
+    groups:
+      docker:
+        patterns: ["*"]
+
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: monthly
+    cooldown:
+      default-days: 7
+    labels: [dependencies]
+    commit-message:
+      prefix: ci
+      include: scope
+    groups:
+      actions:
+        patterns: ["*"]

.github/workflows/dependabot-automerge.yml

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+name: Dependabot auto-merge
+
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  automerge:
+    if: github.event.pull_request.user.login == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - id: meta
+        uses: dependabot/fetch-metadata@v2
+      # Majors stay open for a manual look. The merge waits for the required CI checks.
+      - if: steps.meta.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 }}