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

tests/conftest.py

@@ -1,7 +1,8 @@
 """Shared pytest fixtures for the test suite."""
 from datetime import datetime
-from typing import AsyncGenerator, Generator
+from typing import Any, AsyncGenerator, Callable, Generator
 import pytest
+import fakeredis
 from sqlalchemy import Engine
 from sqlmodel import SQLModel, Session, create_engine
 from httpx import ASGITransport, AsyncClient
@@ -184,3 +185,59 @@ async def async_client(
 
     # Clean up dependency overrides
     app.dependency_overrides.clear()
+
+
+@pytest.fixture
+def fake_redis() -> Generator[fakeredis.FakeRedis, None, None]:
+    """Create a fakeredis client, flushed after each test."""
+    client = fakeredis.FakeRedis(decode_responses=True)
+    yield client
+    client.flushall()
+
+
+@pytest.fixture
+def rent_listing_factory() -> Callable[..., RentListing]:
+    """Factory function that creates RentListing with overridable defaults."""
+    _counter = 0
+
+    def _create(**overrides: Any) -> RentListing:
+        nonlocal _counter
+        _counter += 1
+        defaults: dict[str, Any] = dict(
+            id=_counter,
+            price=2000.0,
+            number_of_bedrooms=2,
+            square_meters=55.0,
+            agency="Test Agency",
+            council_tax_band="C",
+            longitude=-0.1276,
+            latitude=51.5074,
+            price_history_json="[]",
+            listing_site=ListingSite.RIGHTMOVE,
+            last_seen=datetime.now(),
+            photo_thumbnail="https://example.com/photo.jpg",
+            floorplan_image_paths=[],
+            additional_info={"property": {"visible": True}},
+            routing_info_json=None,
+            furnish_type=FurnishType.FURNISHED,
+            available_from=datetime.now(),
+        )
+        defaults.update(overrides)
+        return RentListing(**defaults)
+
+    return _create
+
+
+@pytest.fixture
+async def seeded_repository(
+    in_memory_engine: Engine,
+    rent_listing_factory: Callable[..., RentListing],
+) -> ListingRepository:
+    """Repository with 10 pre-seeded listings (varied price/bedrooms/sqm)."""
+    repo = ListingRepository(engine=in_memory_engine)
+    listings = [
+        rent_listing_factory(id=100 + i, price=1000 + i * 300, number_of_bedrooms=(i % 4) + 1, square_meters=30 + i * 10)
+        for i in range(10)
+    ]
+    await repo.upsert_listings(listings)
+    return repo