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_billing_service.py

@@ -0,0 +1,33 @@
+"""Tests for billing service."""
+import pytest
+from app.domains.billing.service import create_invoice, get_invoices
+
+
+@pytest.mark.asyncio
+async def test_create_invoice_minimal(db, admin_user):
+    """Invoice kann mit Mindestdaten erstellt werden."""
+    invoice = await create_invoice(
+        db,
+        tenant_id=None,
+        order_line_ids=[],
+        notes="Test invoice",
+    )
+    assert invoice.id is not None
+    assert invoice.invoice_number.startswith("INV-")
+    assert invoice.status == "draft"
+    assert invoice.currency == "EUR"
+
+
+@pytest.mark.asyncio
+async def test_invoice_number_sequential(db, admin_user):
+    """Invoice-Nummern sind sequenziell und eindeutig."""
+    inv1 = await create_invoice(db, tenant_id=None, order_line_ids=[], notes="First")
+    inv2 = await create_invoice(db, tenant_id=None, order_line_ids=[], notes="Second")
+    assert inv1.invoice_number != inv2.invoice_number
+
+
+@pytest.mark.asyncio
+async def test_get_invoices_returns_list(db):
+    """get_invoices gibt eine Liste zurück."""
+    invoices = await get_invoices(db, tenant_id=None)
+    assert isinstance(invoices, list)