feat: fix creator key cooldown

[Stanley-Owoh]

Summary

Implement server-side buy cooldown enforcement middleware for creator key purchases to prevent wallets from purchasing the same key again before the configured cooldown period expires.

The middleware checks the wallet's cooldown status before allowing a purchase to proceed, uses Redis for efficient cooldown tracking, and falls back to a database check if Redis is unavailable.

Changes Made

• Cooldown enforcement: Check whether a wallet is still within its cooldown period before processing a creator key purchase.
• Redis-based tracking: Store cooldown expiry timestamps in Redis using a key composed of the wallet address and creator key ID.
• Configurable cooldown duration: Read the cooldown duration for each creator key from its contract metadata.
• Rate-limit response: Reject purchases during an active cooldown with HTTP 429 Too Many Requests, including cooldown_expires_at in the response.
• Cooldown initialization: Set the appropriate cooldown expiry after a successful purchase.
• Database fallback: Fall back to checking the database for existing cooldown information when Redis is unavailable, ensuring cooldown enforcement remains reliable.

Acceptance Criteria

• Purchase requests are rejected with HTTP 429 when the wallet is within its cooldown period.
• Rejection responses include the cooldown_expires_at timestamp.
• Cooldown expiry is set correctly after a successful purchase.
• Cooldown duration is retrieved from the creator key's contract metadata.
• Database fallback is supported when Redis fails.

Expected Outcome

Creator key purchases are protected by consistent server-side cooldown enforcement, preventing wallets from bypassing configured purchase restrictions while maintaining reliable behavior during Redis failures.

Closes #877

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