feat: issues #484 #491 #490 #489 — zero-balance holdings test, alert …

[villadel]

…invalid address test, buildValidationError helper, activity historical price test

Summary

Testing

• pnpm lint
• pnpm build
• pnpm exec prisma generate when schema or generated types changed

Checklist

• Linked issue or backlog item
• No secrets or live credentials added
• Docs updated if setup or env changed
• Change is scoped to one problem
  closes
  closes Add integration test for wallet holdings endpoint excluding zero balance entries #484
  closes Add integration test for activity feed returning price at time of trade not current price #489
  closes Add helper for building a standardized 422 validation error response shape #490
  closes Add integration test for alert registration returning 400 for invalid Stellar wallet address #491

[drips-wave]

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