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>

backend/app/config/settings.py

@@ -212,6 +212,18 @@ class Settings(BaseSettings):
         ),
     )
 
+    # Optional override for the URL shown on the System Status page and copied
+    # into client configs. Needed because request.base_url reflects the Host
+    # header, which the Vite dev proxy (changeOrigin: true) and reverse proxies
+    # that do not forward the original Host both rewrite.
+    mcp_public_url: str = Field(
+        default="",
+        description=(
+            "Externally reachable MCP endpoint URL, e.g. http://6.86.80.9:8000/mcp/. "
+            "Leave empty to derive it from the incoming request."
+        ),
+    )
+
 @lru_cache
 def get_settings() -> Settings:
     """Return settings."""