Remote MCP: enable with rate limiting for public demo

[kiyeonjeon21]

What

MCP remote server is disabled in demo mode (Fly.io). Enable it with rate limiting so people can try:

```json
{
"mcpServers": {
"parsemux": {
"url": "https://parsemux-api.fly.dev/mcp"
}
}
}
```

Options

• A: Enable MCP in demo mode with same rate limit (10 req/min)
• B: Separate MCP endpoint with API key auth
• C: Separate MCP endpoint with token-based auth

Files

• `src/parsemux/core/config.py` — `demo_disable_mcp` setting
• `src/parsemux/api/app.py` — MCP mount logic