Add helper for normalizing creator ID param to consistent type before database lookup

[Chucks1093]

Summary

Creator ID params arrive as strings from the URL but are stored as integers in the database. Each handler currently coerces the type independently. A shared helper that parses and validates a creator ID string should be used at the start of every handler that accepts a creator ID.

Scope

• Add parseCreatorId(raw: string): number helper that parses to integer and throws a 400 error if the value is not a positive integer
• Use the helper in: creator detail, creator holders, creator webhooks, buyback quote endpoints
• Add unit tests for: valid integer string, float string, negative number, non-numeric string, empty string

Acceptance Criteria

• Valid positive integer string parses correctly
• All invalid inputs throw a 400-shaped error
• Helper used consistently across all creator ID params

ETA: 12 hours

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Coordinate on Telegram

[Kappa16]

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[K1NGD4VID]

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[grantfox-oss]

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@K1NGD4VID's PR #501 was approved and merged by @Chucks1093.

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