feat: add creator key trading operations

[Akanimoh12]

Summary

Implements the creator-key operational controls, persistence, price history, and Horizon settlement flow as one cohesive trade lifecycle change.

• Adds JWT-admin protected pause and resume endpoints for creator keys, with audit events.
• Rejects buy and multi-buy requests while a key is paused.
• Persists circulatingSupply from confirmed buy/sell activity with three-attempt exponential retry.
• Adds price-history retrieval at GET /api/v1/keys/:keyId/price-history with validated ranges, supported intervals, and a 500-point cap.
• Adds signed Horizon webhook processing for pending purchase memos and ownership transfer audit events.
• Adds Prisma schema and SQL migration for key operation state and pending purchases.

Validation

• Editor diagnostics reported no errors in the touched TypeScript files.
• pnpm exec prisma generate && pnpm build was attempted, but execution was cancelled by the local environment before it ran.

Closes #777
Closes #781
Closes #782
Closes #783

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