Proxy POST requests for deprecated endpoints in dev env

[bhardwajparth51]

Problem

When a deprecated web.py endpoint receives a POST request in the local dev environment, it fails because the 303 redirect doesn't preserve the request body and headers. This affects any POST request to a deprecated endpoint, currently breaking the "set yearly reading goal" feature on localhost.

Reproducing the bug

1. Start the local dev environment with make git && docker compose up -d
2. Send a POST request to a deprecated endpoint:

curl -i -X POST http://localhost:8080/reading-goal.json \
  -d '{"year":2024}' \
  -H "Content-Type: application/json"

3. Observe that the request fails — the 303 redirect drops the POST body and headers

Expected behavior: POST requests to deprecated endpoints should be forwarded to the fast_web container and return the correct response.
Actual behavior: POST requests fail because the 303 redirect drops the request body and headers, causing the request to not reach the fast_web container correctly.

Context

• OS: Linux
• Environment: (dev/local)
• The fast_web container is accessible internally via http://fast_web:8080
• GET requests to deprecated endpoints work correctly via redirect

Breakdown

Implementation Details (for maintainers)

Requirements Checklist

• [ ]

Related files

Stakeholders

@RayBB

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[RayBB]

Note: this is blocking: #11895