fix(router): handle None temp_c/vram_pct in gpu_health_score

AMD sidecar returns null for temp_c/power_w fields.
gpu_health_score now uses  fallback for all numeric fields.
check_gpu_health also defaults temp_c and gpu_util_pct to 0.

Fixes 500 error on /metrics/gpu-health

router/router.py

@@ -226,13 +226,13 @@ def gpu_health_score(model):
     h = check_gpu_health(model, sidecar_timeout=1.5, gpu_timeout=1)
     if h.get("status") == "down":
         return 999  # never pick down GPUs
-    vram_pct = h.get("vram_pct", 50)
-    temp_c = h.get("temp_c", 50)
+    vram_pct = h.get("vram_pct") or 50
+    temp_c = h.get("temp_c") or 50
     active = gpu_active_count(model)
     max_c = GPU_MAX_CONCURRENT.get(model, 1)
-    load_pct = (active / max_c) * 100
+    load_pct = (active / max_c) * 100 if max_c > 0 else 0
     # Score: lower = better (more headroom, cooler, less loaded)
-    score = vram_pct * 0.4 + max(temp_c - 30, 0) * 0.3 + load_pct * 0.3
+    score = (vram_pct or 0) * 0.4 + max((temp_c or 50) - 30, 0) * 0.3 + load_pct * 0.3
     return score
 
 def select_best_gpu(candidates, reason, agent=""):