feat(L+M): configurable dashboard widget system + test framework

Phase L: Dashboard widget system
- Migration 046: dashboard_configs table (user/tenant/role fallback cascade)
- DashboardConfig model + dashboard_service with get/upsert per-user and tenant-default
- API router: GET/PUT /api/dashboard/config, GET/PUT /api/dashboard/tenant-default
- Frontend: 5 widget components (ProductionStats, QueueStatus, RecentRenders, CostOverview, WorkerStatus)
- DashboardGrid with API-backed config, DashboardCustomizeModal (checkbox selection, save/cancel)
- Dashboard.tsx: widget grid + analytics section (privileged users)
- Admin.tsx: restructured with new section order and Maintenance 2-col grid

Phase M: Test framework
- Backend: pytest-asyncio + pytest-cov + factory-boy in pyproject.toml
- conftest.py: excel_dir fixtures + async DB fixtures + mock storage/celery stubs
- Domain tests: billing_service, cache_service, notifications_service, imports_sanity
- Integration: test_api_health.py smoke test (requires running backend)
- Frontend: vitest + @testing-library/react + msw added to package.json
- vite.config.ts: test block (jsdom + globals + setupFiles)
- tsconfig.json: exclude src/__tests__ from main tsc (test runner handles its own types)
- MSW handlers for /api/auth/me, Billing.test.tsx, formatters.test.ts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/domains/test_imports_sanity.py

@@ -0,0 +1,28 @@
+"""Tests for Excel sanity check service (sync Celery service).
+
+Note: run_sanity_check opens a real synchronous DB connection, so the
+meaningful tests are integration-only (marked accordingly). The unit-level
+test only verifies the import and signature.
+"""
+import pytest
+
+
+def test_run_sanity_check_importable():
+    """run_sanity_check can be imported without errors."""
+    from app.domains.imports.service import run_sanity_check
+    assert callable(run_sanity_check)
+
+
+@pytest.mark.integration
+def test_run_sanity_check_missing_file_does_not_crash():
+    """Sanity-check-Service gibt leeres Dict zurück bei nicht existierendem File (integration)."""
+    from app.domains.imports.service import run_sanity_check
+    import uuid
+    import app.models  # noqa - register all models so SQLAlchemy relationships resolve
+    result = run_sanity_check(
+        validation_id=str(uuid.uuid4()),
+        excel_path="/nonexistent/test.xlsx",
+        tenant_id=None,
+    )
+    # Service returns {} when validation not found
+    assert isinstance(result, dict)