Remove outdated GPU marketplace endpoint and fix staking service logic

- Remove duplicate `/marketplace/gpu/{gpu_id}` endpoint from marketplace_gpu.py
- Remove marketplace_gpu router inclusion from main.py (already included elsewhere)
- Fix staking service staker_count logic to check existing stakes before increment/decrement
- Add minimum stake amount validation (100 AITBC)
- Add proper error handling for stake not found cases
- Fix staking pool update to commit and refresh after modifications
- Update CLI send_transaction to use chain

apps/coordinator-api/src/app/domain/bounty.py

@@ -4,7 +4,7 @@ Database models for AI agent bounty system with ZK-proof verification
 """
 
 import uuid
-from datetime import datetime
+from datetime import datetime, timedelta
 from enum import StrEnum
 from typing import Any