Remove outdated GPU marketplace endpoint and fix staking service logic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

contracts/package.json

@@ -0,0 +1,9 @@
+{
+  "type": "module",
+  "devDependencies": {
+    "@nomicfoundation/hardhat-toolbox": "^6.1.2",
+    "@openzeppelin/contracts": "^4.9.6",
+    "dotenv": "^17.4.2",
+    "hardhat": "^2.28.6"
+  }
+}