feat(backend): add GET /api/freeze/history endpoint for completed freezes

[omolobamoyinoluwa-max]

Summary

Once a quorum is reached the proposal is deleted from the contract and cleared from SQLite. There is no way to query historical freeze events. Add an endpoint that reads FREEZE events from Horizon.

Acceptance criteria

• GET /api/freeze/history returns past FREEZE events from Horizon event stream
• Supports ?asset=RWAUSD filter