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>

CONTRIBUTING.md

@@ -93,6 +93,7 @@ BanGUI/
 | `tsc --noEmit` | Frontend type checking | `cd frontend && tsc --noEmit` |
 | `eslint` | Frontend linting | `cd frontend && eslint src` |
 | `prettier --check` | Frontend formatting | `cd frontend && prettier --check src` |
+| `import-linter` | Layer boundary enforcement | `cd backend && linter` |
 
 **All checks must pass before committing.** CI runs the same suite.