feat: surface MCP server status in System Status page

Add per-tool in-memory call counters to the MCP module and a
GET /api/v1/status/mcp endpoint that joins them with the live tool
registry and endpoint config, then render it as a new card on the
System Status page with a one-click client-config copy button.

- app/mcp/stats.py: lock-guarded MCPStatsTracker (the mcp SDK runs sync
  tool bodies via anyio.to_thread.run_sync, so this is genuinely
  multi-threaded, unlike the async REST routes)
- app/mcp/server.py: instrument search_regulations, add get_mcp_status()
- app/config/settings.py: optional MCP_PUBLIC_URL override, required
  because the Vite proxy and reverse proxies rewrite the Host header
- StatusPage.tsx: MCP Server card, joins the existing parallel fetch

Counters are process-local by design; token usage is already persisted
by ModelUsageTracker since MCP calls route through ask().

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.env.example

@@ -176,3 +176,9 @@ CORS_ALLOW_ORIGINS=http://localhost:5173
 # MCP_ALLOWED_HOSTS=6.86.80.9:*,127.0.0.1:*,localhost:*
 MCP_ALLOWED_HOSTS=127.0.0.1:*,localhost:*,[::1]:*
 
+# 系统状态页展示、以及"复制接入配置"按钮所使用的 MCP 外部访问地址。
+# 留空则由后端从请求的 Host 头推导；当前端经 Vite 代理（changeOrigin: true）
+# 或反向代理改写了 Host 时，推导结果会是 127.0.0.1，此时必须显式指定。
+# MCP_PUBLIC_URL=http://6.86.80.9:8000/mcp/
+MCP_PUBLIC_URL=
+