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/db/models.py

@@ -25,6 +25,7 @@ class User(Base):
     id = Column(String, primary_key=True, default=generate_uuid)
     email = Column(String, unique=True, nullable=False)
     password_hash = Column(String, nullable=False)
+    tier = Column(String, default="free")
     created_at = Column(DateTime, default=datetime.utcnow)
     updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)