Add comprehensive automated testing infrastructure

Backend:
  - pytest configuration with async support and coverage
  - Unit tests for notification service (23 tests)
  - Unit tests for smart plug manager (12 tests)
  - Unit tests for archive service (16 tests)
  - Integration tests for API endpoints
  - Fix: notifications now send immediately (digest is summary only)

  Frontend:
  - Vitest configuration with jsdom and coverage
  - MSW for API mocking
  - Component tests for Toggle, Button, Card, ConfirmModal (77 tests)
  - Test utilities with custom render wrapper

  CI/CD:
  - GitHub Actions workflow for automated testing
  - Backend lint, unit tests, integration tests
  - Frontend lint, type-check, unit tests, build

backend/tests/pytest.ini

@@ -0,0 +1,14 @@
+[pytest]
+testpaths = .
+asyncio_mode = auto
+asyncio_default_fixture_loop_scope = function
+filterwarnings =
+    ignore::DeprecationWarning
+    ignore::sqlalchemy.exc.SAWarning
+    # Filter warnings from async mocks - coroutines created by mocks that are
+    # intentionally not awaited (expected behavior in unit tests)
+    ignore:coroutine.*was never awaited:RuntimeWarning
+markers =
+    unit: Unit tests (fast, no external deps)
+    integration: Integration tests (slower, test full API)
+    slow: Slow tests (skip with -m "not slow")