feat(backend): Implement AVE Cloud integration for Data and Trading APIs

- Add tier field to User model for plan detection (free/normal/pro)
- Create AVE Cloud API client with all Data API endpoints:
  - Token search (GET /v2/tokens)
  - Batch prices (POST /v2/tokens/price)
  - Token details (GET /v2/tokens/{id})
  - Kline data (GET /v2/klines/token/{id})
  - Trending tokens (GET /v2/tokens/trending)
  - Token risk (GET /v2/contracts/{id})
- Add Trading API endpoints:
  - Chain wallet quote (POST /v1/chain/quote)
  - Chain wallet swap (POST /v1/chain/swap)
- Add tier gating with upsell messaging for Pro features
- Handle rate limiting gracefully with 429 responses
- Add Pydantic schemas for AVE API requests/responses

Fixes #11

src/backend/app/services/ave/__init__.py

@@ -0,0 +1,3 @@
+from .client import AveCloudClient, check_tier_access
+
+__all__ = ["AveCloudClient", "check_tier_access"]