Remove outdated GPU marketplace endpoint and fix staking service logic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/main.py

@@ -301,7 +301,6 @@ def create_app() -> FastAPI:
     app.include_router(edge_gpu)
 
     # Add standalone routers for tasks and payments
-    app.include_router(marketplace_gpu, prefix="/v1")
 
     if ml_zk_proofs:
         app.include_router(ml_zk_proofs)