OCDI-303: Fix SPV asset controller with MongoDB retry logic

[urbantech]

This PR addresses the MongoDB connection issues in the SPV asset controller by implementing proper retry logic and error handling.

Changes made:

1. Removed duplicate SPVAsset model import
2. Added MongoDB retry logic using mongoDbConnection.withRetry
3. Implemented consistent document serialization with toObject
4. Fixed response data handling and error messages
5. Added proper debug logging through res.locals

These changes follow the Semantic Seed Coding Standards V2.0 and improve the reliability of SPV asset operations by properly handling MongoDB connections and document serialization.

Testing:

• All MongoDB operations now use retry logic
• Response data is properly serialized
• Error handling is consistent across all endpoints

Related to PR #158