P1a: Fix MapMcp to use explicit /mcp route pattern

The parameterless MapMcp() overload did not register the endpoint
correctly at the /mcp path. Changed to MapMcp("/mcp") which
successfully registers the MCP Streamable HTTP endpoint.

Verified:
- POST /mcp with initialize request returns valid MCP response
- tools/list returns all 10 NexusMcpTools
- Health check passes
- Build: 0w 0e

backend/Program.cs

@@ -22,6 +22,6 @@ await app.EnsureDatabaseAsync();
 // --- Middleware Pipeline ---
 app.UseNexusPipeline(app.Environment);
 
-app.MapMcp();
+app.MapMcp("/mcp");
 app.MapControllers();
 app.Run();