Add comprehensive test suite: 219 new tests across backend and frontend

Backend (103 tests):
- Unit tests for listing_service, export_service, district_service
- Regression tests for API response contracts and query parameter validation
- Integration tests for API workflows, Redis listing cache, listing processor pipeline, and repository advanced queries
- E2E tests for streaming with filters, batching, caching, and task management

Frontend (116 tests):
- Service tests for apiClient, streamingService, taskService, listingService, healthService
- Hook tests for useTaskProgress (WebSocket + polling)
- Component tests for PropertyCard, FilterPanel, Header, ListView, TaskProgressDrawer, TaskIndicator, StreamingProgressBar, HealthIndicator
- E2E tests for filter-stream-display flow

Infrastructure:
- Add pytest-xdist and test markers (regression, integration, e2e)
- Add conftest fixtures: fake_redis, rent_listing_factory, seeded_repository
- Add vitest + testing-library + MSW for frontend testing

pyproject.toml

@@ -46,6 +46,7 @@ pytest-cov = "^4.1.0"
 httpx = "^0.27.0"
 aioresponses = "^0.7.6"
 fakeredis = "^2.21.0"
+pytest-xdist = "^3.5.0"
 mypy = "^1.8.0"
 types-requests = "^2.31.0"
 types-redis = "^4.6.0"
@@ -65,6 +66,11 @@ exclude = ["*.ipynb"]
 asyncio_mode = "auto"
 testpaths = ["tests"]
 asyncio_default_fixture_loop_scope = "function"
+markers = [
+    "regression: locks down existing behavior",
+    "integration: tests multiple components together",
+    "e2e: end-to-end workflow tests",
+]
 
 [tool.mypy]
 python_version = "3.11"