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/requirements.txt

@@ -3,5 +3,4 @@ fastapi>=0.115.0,<1.0.0
 uvicorn[standard]>=0.30.0,<1.0.0
 httpx>=0.27.0,<1.0.0
 structlog>=24.0.0,<25.0.0
-cryptography>=42.0.0,<44.0.0
-prometheus_client>=0.20.0,<1.0.0
+cryptography>=42.0.0,<44.0.0