Community Forum API

Problem/Feature Description

A startup is building a community forum. The frontend is a React single-page application hosted at https://forum.example.com, and the backend is a new FastAPI service on a separate domain. Users will post questions and answers through the frontend, which calls the API.

The same codebase needs to run in staging and production without code changes -- origins and hostnames differ per environment and should come from environment variables.

Output Specification

Produce a single Python file named main.py that:

• Defines a FastAPI application
• Includes at least two example API routes (e.g., GET /posts and POST /posts)
• The frontend at https://forum.example.com must be able to call the API from the browser

The file should be complete and runnable (all imports present). You do not need to implement real database logic -- stubs or placeholder return values are fine.