Stream paginated report exports

[muokwejosh-cloud]

Closes #182

Summary

• Stream admin report and ledger CSV exports from bounded database cursors.
• Use stable timestamp/ID ordering to avoid pagination gaps or duplicates.
• Add reusable CSV stream helper with backpressure support.
• Add streaming audit-event and audit-CSV APIs.

Validation

• git diff --check passed.
• Typecheck was not run because project dependencies are not installed (tsc unavailable).

Affected Areas

• Frontend
• Backend
• Authentication
• Payments
• Stellar
• Documentation
• Tests
• UI/UX

Checklist

• Code follows project conventions
• npm run lint passes
• npm run build passes
• Tests updated where needed
• No secrets committed
• No production credentials used
• Documentation updated if required

[Obiajulu-gif]

Thanks for the contribution! Merging this now as part of a maintainer PR sweep.