Python HTTPS Middleware with Bearer

I need a small, self-contained middleware layer written in Python that can sit inside my server application and act as an HTTPS translator. The task is straightforward: when the server receives an inbound HTTPS request carrying a JSON payload, the middleware should reshape or forward that payload, attach a Bearer token to the outgoing headers, and send the call on to a target REST API.

I am already committed to using Python, but the specific micro-framework is flexible—Flask, FastAPI, or even bare-bones WSGI are all acceptable so long as the final code is clean and easy to drop into an existing service. The incoming data will always be JSON, so request parsing should be trivial. Robust error handling, minimal logging, and simple configuration (token value, target URL, optional header mapping) are more important to me than fancy abstractions.

Deliverables
• Fully functioning Python middleware file(s) ready to import
• Basic README that shows installation, configuration of the Bearer token, and an example curl test
• One or two concise unit tests proving the token is added and the external API is reached successfully

If you keep the code lightweight, clear, and well-commented, this should be a fast win for both of us.