fix(sidecar-v2): second-round review fixes

- cooldown_manager: move function-level imports to module top
- proxy.py: emergency_count counter now actually increments
- server.py: metrics reads emergency_count from proxy module
- dashboard.html: real JS CDN fallback (not just comment)
- requirements.txt: remove unused prometheus_client

Round 2 review residual fixes from 沈路明/陆怀瑾/梁思筑 feedback

Co-authored-by: multica-agent <github@multica.ai>

services/nvidia_sidecar/proxy.py

@@ -18,6 +18,14 @@ from cooldown_manager import start_cooldown, check_and_clear_cooldown
 from storage.models import Backend
 from storage.usage_store import record_usage
 
+# Emergency activation counter (read by metrics endpoint)
+_emergency_count: int = 0
+
+
+def get_emergency_count() -> int:
+    return _emergency_count
+
+
 logger: structlog.stdlib.BoundLogger = structlog.get_logger("sidecar_v2.proxy")
 
 
@@ -299,6 +307,9 @@ async def handle_proxy_request(
         emergency_rpm=emergency_rpm,
     )
 
+    # Track emergency activation for metrics
+    _emergency_count += 1
+
     # Emergency: try to get a token from any fallback backend at reduced RPM
     emergency_retries = 3
     for attempt in range(emergency_retries):