Switch MCP transport from stdio to Streamable HTTP

Mount POST /mcp on the main Express app (stateless Streamable HTTP).
Keep optional MCP-only HTTP process via npm run mcp. Update docs.

__tests__/mcp-server.test.js

@@ -1,4 +1,6 @@
+const express = require('express');
 const { formatSize, createWebshareMcpServer } = require('../src/mcp-server');
+const { registerMcpHttp } = require('../src/mcp-http');
 
 describe('formatSize', () => {
   it('formats bytes', () => {
@@ -17,3 +19,14 @@ describe('createWebshareMcpServer', () => {
     expect(server.server).toBeDefined();
   });
 });
+
+describe('registerMcpHttp', () => {
+  it('registers POST /mcp on an express app', () => {
+    const app = express();
+    registerMcpHttp(app, { username: 'u', password: 'p', path: '/mcp' });
+    const layer = app._router.stack.find(
+      (l) => l.route && l.route.path === '/mcp' && l.route.methods.post
+    );
+    expect(layer).toBeTruthy();
+  });
+});