feat(backend): add deprecation middleware and API versioning support

- Add deprecation middleware for warning headers on sunset endpoints
- Add jails_v2 router for API v2 migration path
- Update CI workflow with new test coverage
- Update API versioning documentation
- Remove completed tasks from Tasks.md

.github/workflows/ci.yml

@@ -98,4 +98,77 @@ jobs:
         run: pip install -e ".[dev]"
 
       - name: Run import-linter
-        run: linter
+        run: linter
+
+  openapi-breaking-changes:
+    name: OpenAPI Breaking Changes
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: backend
+    # Only run on PRs — main branch push is covered by the baseline-commit step.
+    if: github.event_name == 'pull_request'
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install dependencies
+        run: pip install -e ".[dev]"
+
+      - name: Generate current OpenAPI spec
+        run: python scripts/generate_openapi.py current-openapi.json
+
+      - name: Fetch baseline spec from main
+        run: |
+          git fetch origin main:main
+          git show main:backend/openapi.json > baseline-openapi.json 2>/dev/null || \
+            echo "{}" > baseline-openapi.json
+
+      - name: Install openapi-diff
+        run: npm install -g openapi-diff
+
+      - name: Check for breaking changes
+        run: |
+          set +e
+          openapi-diff baseline-openapi.json current-openapi.json --format stylish 2>&1
+          EXIT_CODE=$?
+          if [ $EXIT_CODE -ne 0 ]; then
+            echo "BREAKING CHANGE DETECTED — see output above"
+            exit 1
+          fi
+          echo "No breaking changes found."
+
+  openapi-baseline-commit:
+    name: OpenAPI Baseline Commit
+    runs-on: ubuntu-latest
+    # Only run on push to main (not PRs).
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install dependencies
+        run: pip install -e ".[dev]"
+
+      - name: Generate and commit OpenAPI baseline
+        run: |
+          python scripts/generate_openapi.py backend/openapi.json
+
+          git config --local user.email "github-actions[bot]@users.noreply.github.com"
+          git config --local user.name "github-actions[bot]"
+
+          git add backend/openapi.json
+          git diff --cached --quiet && echo "No changes to openapi.json" || \
+            git commit -m "chore: update OpenAPI baseline spec [skip ci]
+
+Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>"