feat: add invoice URL to proration and test seed scripts

[sean-brydon]

• Add invoiceUrl field to MonthlyProration schema
• Update billing service to return hosted_invoice_url after finalizing
• Save invoice URL when creating proration invoices
• Add seed script for testing proration with real Stripe data
• Add cleanup script for test data removal

What does this PR do?

• Fixes #XXXX (GitHub issue number)
• Fixes CAL-XXXX (Linear issue number - should be visible at the bottom of the GitHub issue description)

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Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

• Are there environment variables that should be set?
• What are the minimal test data to have?
• What is expected (happy path) to have (input and output)?
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Checklist

• I haven't read the contributing guide
• My code doesn't follow the style guidelines of this project
• I haven't commented my code, particularly in hard-to-understand areas
• I haven't checked if my changes generate no new warnings
• My PR is too large (>500 lines or >10 files) and should be split into smaller PRs

[cubic-dev-ai]

No issues found across 8 files

[keithwillcode]

Looks good from the DB side.

[github-actions]

E2E results are ready!

• Workflow #101262.2 latest results