feat(x402): FastAPI/Flask Python decorator + Express.js TypeScript middleware

[jdjioe5-cpu]

Two x402 packages targeting the GrantFox OSS Campaign

This PR adds two complementary x402 implementations for the Nirium SDK v0.7.0, one in Python (FastAPI / Flask) and one in TypeScript (Express.js). Both satisfy the acceptance criteria from the GrantFox bounty campaign (Org ID: b6550049-a239-4dc1-b95e-2fb28ded7ea6).

1. Python - nirium.fastapi_x402

from fastapi import FastAPI, Request
from nirium.fastapi_x402 import x402_required

app = FastAPI()

@app.get("/premium")
@x402_required(price="0.02", pay_to="GXXX_STELLAR_ADDRESS")
async def premium(request: Request):
    return {"data": "paid-only"}

• One decorator works on FastAPI and Flask routes.
• Inspects X-402-Signature header; returns HTTP 402 + structured body + WWW-Authenticate when missing or invalid.
• Async verify via aiohttp with bounded timeout.
• Graceful bounded fallback when settlement is unreachable.
• 5/5 pytest cases pass.

2. TypeScript - @nirium/express-x402

import express from 'express';
import { nirium } from '@nirium/express-x402';

app.get('/premium',
  nirium.x402Serve({ price: '0.02', payTo: 'GXXX_STELLAR_ADDRESS' }),
  (req, res) => res.json({ ok: true, paid: req.x402Validation })
);

• Exposes nirium.x402Serve matching the Python helper API.
• Uses fetch + AbortController for bounded verify.
• Returns HTTP 402 + structured body + headers when missing/invalid signature.
• 5/5 jest cases pass.

Bounded by design

Both packages validate signatures only. They never open bank rails, never issue credits, never mutate external state, never sign or broadcast Stellar transactions.

Tests

# Python
cd packages/sdk-python
pip install -e .
pip install fastapi httpx pytest
python -m pytest tests/test_fastapi_x402.py -v

# TypeScript
cd packages/sdk/express-x402
npm install
npm test

[jdjioe5-cpu]

Hi maintainer — ping for review on PR #28. Both bounty #9 (Express.js) and #10 (Python) deliverables are complete:

• Python: nirium.fastapi_x402 (5/5 pytest pass) — FastAPI/Flask @x402_required decorator
• TypeScript: @nirium/express-x402 (5/5 jest pass) — nirium.x402Serve middleware

PR is mergeable: clean. No conflicts with main. Ready for review whenever you have a moment. Both deliverables satisfy the bounty acceptance criteria:

#10 (FastAPI Python x402 Decorator):

• Compatible with FastAPI & Flask (single decorator)
• Inspects X-402-Signature header and validates settlement
• Exposes clean 402 HTTP status codes
• Includes unit tests

#9 (Express.js x402 Micropayment Middleware Helper):

• Exposes nirium.x402Serve({ price: '0.02', payTo: 'STELLAR_ADDRESS' })
• Returns HTTP 402 Payment Required when no valid signature is provided
• Includes unit tests with Jest

Payout address: Base USDC (will DM on completion). Thanks!