fix(BIZ-42): critical deadlock + major review issues

- Fix #1 (Critical): evaluate_retreat() deadlock — changed _retreat_lock to
  threading.RLock() to allow reentrant acquisition when evaluate_retreat()
  calls get_429_rate() while holding the lock
- Fix #2 (Major): Dashboard queue chart now uses snap.queue.per_priority
  instead of Math.random() mock data
- Fix #3 (Major): structlog uses JSONRenderer instead of ConsoleRenderer
  for JSON-format output as required by acceptance criteria
- Bonus: webui.py _build_snapshot() now async and includes queue data
  with per-priority depth for dashboard consumption

Reviewed-by: 梁思筑 (architect)
Co-authored-by: multica-agent <github@multica.ai>

services/nvidia_sidecar/server.py

@@ -52,8 +52,7 @@ structlog.configure(
         structlog.processors.StackInfoRenderer(),
         structlog.processors.format_exc_info,
         structlog.processors.UnicodeDecoder(),
-        # 生产环境推荐 JSONRenderer，开发环境可用 ConsoleRenderer
-        structlog.dev.ConsoleRenderer(),
+        structlog.processors.JSONRenderer(),
     ],
     context_class=dict,
     logger_factory=structlog.PrintLoggerFactory(),