[SheyeJDev]

Summary

Adds documentation comments to the WhatsApp user service. This is a comment-only change with no functional or behavioral differences.

Changes

• Added doc comments to all exported types and functions, including behavior on missing users, return values, and balance clamping.
• Added inline comments explaining the less obvious logic:
  • OTP lifecycle: when it is cleared, when it is kept on a wrong guess, and single-use consumption.
  • The yield summary's APY calculation: snapshots first, falling back to the latest protocol rates per (protocol, asset) pair.
  • The 30-day reporting window and the parallel snapshot/rate queries.
• Labeled the test-only helpers.

Notes for reviewers

While documenting, I added NOTE comments on two existing issues. I did not fix them here:

• generateOtp uses Math.random(), which isn't cryptographically secure. crypto.randomInt would be a safer choice.
• verifyOtp doesn't limit failed attempts, so a 6-digit code could be brute-forced within its 5-minute TTL.

Also worth knowing: decrementBalance silently clamps to 0 on overdraw instead of throwing.

I'm happy to open follow-up PRs for these.

Testing

No code changes, so no new tests. Existing tests and the build are unaffected.

Closes #660
Closes #661
Closes #659
Closes #679