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>

docs/architecture/backend-project-architecture.md

@@ -322,17 +322,20 @@ backend/app/
 - 以 Model Context Protocol 对外暴露平台已有能力
 - MCP tool 注册与入参 schema 绑定
 - MCP 专用鉴权（复用现有 JWT）与 Streamable HTTP 子 ASGI 应用装配
+- MCP 传输自身的可观测性：进程内 per-tool 调用计数（`stats.py`），以及供 System Status 页面读取的状态汇总 `get_mcp_status()`
 
 非职责：
 
 - 不实现任何新的检索、问答或业务编排逻辑
 - 不直接访问 Milvus、MinIO、LLM SDK
+- 不统计 token 消耗 —— MCP 调用经由 `AgentConversationService.ask()`，已由 `shared/model_usage_tracker.py` 记账
 
 说明：
 
 - `mcp` 与 `api` 是并列的两个 transport 适配层：`api` 面向 HTTP REST 客户端，`mcp` 面向 MCP 客户端（Claude Desktop、IDE 等）。二者共用同一套 application service。
 - 它独立成顶层模块而不是放进 `api/routes/`，因为 MCP 使用装饰器式 tool 注册和自带的子 ASGI 应用，与 `APIRouter` 是不同的传输机制。
 - 当前实现见 `backend/app/mcp/server.py`，只暴露 `search_regulations` 一个 tool，内部直接调用 `get_agent_conversation_service()`。
+- `api/routes/status.py` 的 `GET /status/mcp` 是薄适配层：它只负责解析对外可达的 URL（`settings.mcp_public_url` 或从请求推导），其余全部交给 `mcp.server.get_mcp_status()`，路由不得直接读取 MCP 内部结构。
 
 ## 5. Module Responsibilities