Add integration test for key holders list returning correct held_since for each wallet

[Chucks1093]

Summary

The held_since field on each holder entry must reflect the timestamp of that wallet's first buy event for the creator. An integration test with multiple holders who bought at different times should confirm the field is populated correctly for each one.

Scope

• Seed three wallets with buy events at different timestamps for the same creator
• Call GET /creators/:id/holders and assert each wallet's held_since matches its first buy timestamp
• Assert a wallet that bought twice shows the timestamp of its first buy, not its most recent

Acceptance Criteria

• Each holder's held_since matches their first buy timestamp
• Wallet with multiple buys shows the earliest buy timestamp
• Timestamps are returned in ISO 8601 format

ETA: 12 hours

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Coordinate on Telegram

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