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/app/domains/admin/models.py

@@ -0,0 +1,42 @@
+import uuid
+from datetime import datetime
+from typing import TYPE_CHECKING
+
+from sqlalchemy import Boolean, DateTime, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+
+from app.database import Base
+
+if TYPE_CHECKING:
+    pass
+
+
+class DashboardConfig(Base):
+    __tablename__ = "dashboard_configs"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("tenants.id", ondelete="CASCADE"),
+        nullable=True,
+        index=True,
+    )
+    user_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=True,
+        index=True,
+    )
+    widgets: Mapped[list] = mapped_column(JSONB, nullable=False, default=list)
+    is_tenant_default: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=False
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime, nullable=False, default=datetime.utcnow
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime, nullable=False, default=datetime.utcnow, onupdate=datetime.utcnow
+    )