phsl / new-api

🔧 **fix(model, controller): robust serialization for `ChannelInfo` & correct Vertex-AI key storage**

Summary
1. **model/channel.go**
   • Replaced the pointer-only `Value()` with a value-receiver implementation
   • GORM can now marshal both `ChannelInfo` and `*ChannelInfo`, eliminating
     `unsupported type model.ChannelInfo` runtime error.

2. **controller/channel.go**
   • Refactored `getVertexArrayKeys()` – every element is now
     - passed through `json.Marshal` when not already a string
     - trimmed & validated before insertion
   • Guarantees each service-account key is persisted as a **pure JSON string**
     instead of the previous `map[...]` dump.

Result
• Channel creation / update succeeds without SQL driver errors.
• Vertex-AI batch & multi-key uploads are stored in canonical JSON, ready for
  downstream SDKs to consume.