feat: migrate to Pydantic V2 and implement rate limiting middleware

- Migrate settings.py to Pydantic V2 (SettingsConfigDict, validation_alias)
- Update config models to use @field_validator with @classmethod
- Replace deprecated datetime.utcnow() with datetime.now(timezone.utc)
- Migrate FastAPI app from @app.on_event to lifespan context manager
- Implement comprehensive rate limiting middleware with:
  * Endpoint-specific rate limits (login: 5/min, register: 3/min)
  * IP-based and user-based tracking
  * Authenticated user multiplier (2x limits)
  * Bypass paths for health, docs, static, websocket endpoints
  * Rate limit headers in responses
- Add 13 comprehensive tests for rate limiting (all passing)
- Update instructions.md to mark completed tasks
- Fix asyncio.create_task usage in anime_service.py

All 714 tests passing. No deprecation warnings.

tests/integration/test_websocket_integration.py

@@ -220,7 +220,7 @@ class TestWebSocketDownloadIntegration:
         download_service.set_broadcast_callback(mock_broadcast)
 
         # Manually add a completed item to test
-        from datetime import datetime
+        from datetime import datetime, timezone
 
         from src.server.models.download import DownloadItem
 
@@ -231,7 +231,7 @@ class TestWebSocketDownloadIntegration:
             episode=EpisodeIdentifier(season=1, episode=1),
             status=DownloadStatus.COMPLETED,
             priority=DownloadPriority.NORMAL,
-            added_at=datetime.utcnow(),
+            added_at=datetime.now(timezone.utc),
         )
         download_service._completed_items.append(completed_item)