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>

frontend/src/api/status.ts

@@ -1,4 +1,4 @@
-import { fetchAPI, type ModelUsageResponse, type SystemConfig, type SystemHealth, type SystemStats } from './index';
+import { fetchAPI, type MCPStatusResponse, type ModelUsageResponse, type SystemConfig, type SystemHealth, type SystemStats } from './index';
 
 export async function getSystemStats(): Promise<SystemStats> {
   return fetchAPI<SystemStats>('/status/stats');
@@ -22,4 +22,9 @@ export async function pingModelConnections(): Promise<ModelUsageResponse> {
   return fetchAPI<ModelUsageResponse>('/status/models/ping', { method: 'POST' });
 }
 
-export type { ModelUsageResponse, SystemConfig, SystemHealth, SystemStats };
+/** MCP endpoint config, advertised tools, and per-tool call counters. */
+export async function getMCPStatus(): Promise<MCPStatusResponse> {
+  return fetchAPI<MCPStatusResponse>('/status/mcp');
+}
+
+export type { MCPStatusResponse, ModelUsageResponse, SystemConfig, SystemHealth, SystemStats };