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

frontend/src/__tests__/mocks/handlers.ts

@@ -0,0 +1,77 @@
+import { http, HttpResponse } from 'msw';
+
+export const handlers = [
+  // Health check
+  http.get('/api/status', () => {
+    return HttpResponse.json({ status: 'OK' });
+  }),
+
+  // Get listings
+  http.get('/api/listing', () => {
+    return HttpResponse.json({ listings: [] });
+  }),
+
+  // Get listing GeoJSON
+  http.get('/api/listing_geojson', () => {
+    return HttpResponse.json({
+      type: 'FeatureCollection',
+      features: [],
+    });
+  }),
+
+  // Stream listing GeoJSON
+  http.get('/api/listing_geojson/stream', () => {
+    const lines = [
+      JSON.stringify({ type: 'metadata', batch_size: 50, total_expected: 0, cached: false }),
+      JSON.stringify({ type: 'complete', total: 0 }),
+    ].join('\n') + '\n';
+
+    return new HttpResponse(lines, {
+      headers: { 'Content-Type': 'application/x-ndjson' },
+    });
+  }),
+
+  // Refresh listings
+  http.post('/api/refresh_listings', () => {
+    return HttpResponse.json({ task_id: 'test-task-123', message: 'Task started' });
+  }),
+
+  // Task status
+  http.get('/api/task_status', () => {
+    return HttpResponse.json({
+      task_id: 'test-task-123',
+      status: 'PENDING',
+      result: null,
+      progress: null,
+      processed: null,
+      total: null,
+      message: null,
+      error: null,
+      traceback: null,
+    });
+  }),
+
+  // Tasks for user
+  http.get('/api/tasks_for_user', () => {
+    return HttpResponse.json([]);
+  }),
+
+  // Cancel task
+  http.post('/api/cancel_task', () => {
+    return HttpResponse.json({ success: true, message: 'Task cancelled' });
+  }),
+
+  // Clear all tasks
+  http.post('/api/clear_all_tasks', () => {
+    return HttpResponse.json({ success: true, count: 0, message: 'Cleared 0 tasks' });
+  }),
+
+  // Districts
+  http.get('/api/get_districts', () => {
+    return HttpResponse.json({
+      London: 'REGION^87490',
+      Westminster: 'REGION^93980',
+      Camden: 'REGION^93941',
+    });
+  }),
+];

frontend/src/__tests__/mocks/server.ts

@@ -0,0 +1,4 @@
+import { setupServer } from 'msw/node';
+import { handlers } from './handlers';
+
+export const server = setupServer(...handlers);