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/unit/test_download_models.py

@@ -221,7 +221,7 @@ class TestDownloadItem:
     def test_download_item_with_timestamps(self):
         """Test download item with timestamp fields."""
         episode = EpisodeIdentifier(season=1, episode=1)
-        now = datetime.utcnow()
+        now = datetime.now(timezone.utc)
         item = DownloadItem(
             id="test_id",
             serie_id="serie_id",