feat(workers): implement billing worker, tests, docs (issue #323)

[boalambo]

Closes #323

---

Summary

This PR implements a billing worker that processes billing events,
calculates invoice data, and enqueues billing tasks.

Changes

• Added billing worker implementation
• Added invoice calculation logic
• Added billing task enqueueing
• Added unit tests
• Added integration tests
• Updated CI configuration
• Added documentation for running and testing the worker

Motivation

Implements the billing processing infrastructure requested in #323.
This enables automated subscription and billing workflows.

Testing

• Unit tests pass
• Integration tests pass
• Verified billing event processing locally

[drips-wave]

@boalambo Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[Oluwaseyi89]

@boalambo I will not accept this PR message. It lacks description.

[boalambo]

@boalambo I will not accept this PR message. It lacks description.

I will fix that now

[boalambo]

@boalambo I will not accept this PR message. It lacks description.

Kindly check now. Thank you

[Oluwaseyi89]

@boalambo why did you modify a workflow file? Revert it back to its original state by copying it from the repo main branch.

[boalambo]

Revert it back to its original state by copying it from the repo main branch.

Workflow restore to the original state