fix: replace deprecated datetime.utcnow() with datetime.now(datetime.UTC)

- Replace all 2,087 uses of datetime.utcnow() across 294 files
- Add UTC import to datetime statements where needed
- Addresses Python 3.12+ deprecation warning (report item #3)

dev/gpu/gpu_miner_host.py

@@ -10,7 +10,7 @@ import logging
 import sys
 import subprocess
 import os
-from datetime import datetime
+from datetime import datetime, UTC
 from typing import Dict, Optional
 
 # Configuration
@@ -211,7 +211,7 @@ def send_heartbeat():
         heartbeat_data = {
             "status": "active",
             "current_jobs": 0,
-            "last_seen": datetime.utcnow().isoformat(),
+            "last_seen": datetime.now(datetime.UTC).isoformat(),
             "gpu_utilization": gpu_info["utilization"],
             "memory_used": gpu_info["memory_used"],
             "memory_total": gpu_info["memory_total"],
@@ -223,7 +223,7 @@ def send_heartbeat():
         heartbeat_data = {
             "status": "active",
             "current_jobs": 0,
-            "last_seen": datetime.utcnow().isoformat(),
+            "last_seen": datetime.now(datetime.UTC).isoformat(),
             "gpu_utilization": 0,
             "memory_used": 0,
             "memory_total": 0,