feat: add computePriceChange helper

[Godfrey-Delight]

Summary

• Adds a CreatorPriceHistory model (id, creatorId, price, recordedAt, indexed on creatorId + recordedAt) and a computePriceChange(creatorId, windowMs, db) helper that fetches the oldest and latest snapshot in a given window and returns the signed percentage change rounded to two decimal places, with 5 unit tests covering positive/negative change, single-snapshot and no-snapshot null cases, and rounding precision. Wires up price-snapshot.service.ts to write a history record on every trade, and switches creator-profile.service.ts and creator-list-item.mapper.ts to use the helper for the 24h window, falling back to the existing inline calc when no history exists yet; the mapper and downstream serializers/controllers (creators.serializers.ts, creators.utils.ts, creators.controllers.ts) are updated to async to support this. Updates existing tests to match the new shapes and mocks.

Closes #643

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

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