Implement Apple JWS notification verification

[singhaditya21]

Implemented cryptographic verification for Apple App Store Server Notifications V2. This includes:

1. Fetching and storing the Apple Root CA - G3 certificate.
2. Implementing a robust verification utility that checks:
   • JWS header x5c presence and algorithm (ES256).
   • Certificate chain validity (each cert signed by the next).
   • Chain trust anchor (last cert signed by Apple Root CA).
   • JWS signature using the leaf certificate's public key.
3. Updating the webhook handler to enforce verification before processing notifications.
4. Adding a comprehensive test suite that generates a temporary PKI to verify the logic against valid and invalid chains/signatures.

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PR created automatically by Jules for task 15564359096076898345 started by @singhaditya21

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[singhaditya21]

Reviewed, leaving open. This is the right thing to do — the Apple webhook currently trusts unverified JWS payloads, which is a real security gap.

Two notes:

1. It commits AppleRootCA-G3.pem. That is a public root certificate so it is not sensitive, but pinning a root in the repo means a silent expiry risk later — worth a comment in the file recording where it came from and when it expires.
2. backend/payment-service has no test runner wired into CI. The test-apple-verifier.js script here only helps if something runs it; the workflow currently covers the Python API only.

I have not merged it because I cannot exercise the verification path without real Apple notification payloads, and getting signature verification subtly wrong is worse than not having it.