fix(sidecar-v2): incorporate review feedback - P0/P1 fixes

P0 fixes:
- Admin API Bearer Token auth middleware
- Encryption key missing -> CRITICAL log + sys.exit(1)
- Prometheus metrics endpoint (:9191)
- requirements.txt + Dockerfile + docker-compose.yml + systemd + nginx

P1 fixes:
- Dead code removed from _refresh_cooldowns()
- Stream detection fixed (text/event-stream only)
- Emergency passthrough (10% RPM retry before 503)
- Active health probing for backends
- SQLite daily backup loop with retention
- Chart.js CDN fallback
- Key rotation SOP document
- JSON log format support
- Deploy files: systemd unit + nginx config

BIZ-52 review re-entry

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

services/nvidia_sidecar/requirements.txt

@@ -0,0 +1,7 @@
+# Sidecar V2 — Multi-Pool Provider Proxy
+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