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_notifications_service.py

@@ -0,0 +1,27 @@
+"""Tests for notification config service."""
+import pytest
+from app.domains.notifications.service import (
+    upsert_notification_config,
+    get_notification_configs,
+)
+
+
+@pytest.mark.asyncio
+async def test_upsert_creates_config(db, admin_user):
+    """Kann Notification-Config anlegen."""
+    await upsert_notification_config(db, admin_user.id, "render_complete", "in_app", True)
+    configs = await get_notification_configs(db, admin_user.id)
+    assert len(configs) >= 1
+    render_cfg = next((c for c in configs if c.event_type == "render_complete"), None)
+    assert render_cfg is not None
+
+
+@pytest.mark.asyncio
+async def test_upsert_updates_existing(db, admin_user):
+    """Update überschreibt bestehende Config."""
+    await upsert_notification_config(db, admin_user.id, "order_submitted", "in_app", True)
+    await upsert_notification_config(db, admin_user.id, "order_submitted", "in_app", False)
+    configs = await get_notification_configs(db, admin_user.id)
+    cfg = next((c for c in configs if c.event_type == "order_submitted"), None)
+    assert cfg is not None
+    assert cfg.enabled is False