Add a recipes/ folder with FastAPI middleware example

[jieyao-MilestoneHub]

What

Add a recipes/fastapi_middleware.py example showing how to use redact_url_query inside a FastAPI middleware that logs request URLs without leaking secret query params.

Expected outcome

• New file recipes/fastapi_middleware.py (under 50 lines)
• Drop-in RedactingLogMiddleware class that:
  • Reads the request URL
  • Passes it through redact_url_query
  • Logs the redacted form before calling await call_next(request)
• README "Three killer examples" section gets a fourth bullet linking to the recipe
• No tests required (it's an integration example, not library code)

Why this is good for newcomers

• Doesn't touch any library internals
• Pattern is well-defined in FastAPI docs
• Showcases a real-world use case redactkit users will want to copy-paste

Pointers

• Source: src/redactkit/url_query.py
• README example block to mirror: ## Three killer examples