Add import-linter boundary to forbid routers importing app.dependencies

Issue #51: Enforce repository boundary at CI level using import-linter.
Contract 'forbid_router_db_import' checks that app.routers never imports
app.dependencies directly, keeping the DB access path through service
contexts only.

- Add import-linter>=2.0.0 to dev dependencies (backend/pyproject.toml)
- Configure [tool.importlinter] with package_root and root_packages
- Add [[tool.importlinter.contracts]] with type='forbidden', source
  app.routers, forbidden app.dependencies
- Add 'Import Boundary' CI job (import-linter)
- Document import-linter in CONTRIBUTING.md code quality table

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/ci.yml

@@ -77,4 +77,25 @@ jobs:
         run: pip install -e ".[dev]"
 
       - name: Run mypy
-        run: mypy app
+        run: mypy app
+
+  import-linter:
+    name: Import Boundary
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: backend
+
+    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: Run import-linter
+        run: linter