Add price alert webhook registration for creator key price thresholds

[Chucks1093]

Summary

Fans who want to buy into a creator at a specific price have no way to know when that price is reached without manually checking. A price alert system lets a fan register a target price and a callback URL, and the server fires the webhook when the creator's key price crosses that threshold. This turns passive watchers into active buyers at the right moment.

Scope

• Add POST /alerts to register an alert: { creator_id, wallet_address, target_price, direction, callback_url }
  • direction is either above (fire when price rises past target) or below (fire when price drops below target)
• Add DELETE /alerts/:id to cancel a registered alert
• When the indexer processes a trade event and the new price crosses any registered threshold, fire the registered callback_url via HTTP POST with payload: { creator_id, triggered_price, target_price, direction, timestamp }
• A fired alert is automatically deleted after delivery (one-shot)
• Failed webhook delivery is retried up to 3 times with exponential backoff

Acceptance Criteria

• Alert registration returns a unique alert ID
• Alert fires when price moves in the registered direction past the target
• Alert does not fire if price moves in the opposite direction
• Alert is deleted after successful delivery
• Failed delivery is retried up to 3 times before the alert is marked failed
• DELETE /alerts/:id cancels a pending alert before it fires
• Integration test covers: above trigger, below trigger, cancellation, failed delivery retry

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Coordinate on Telegram

[bamiebot-maker]

I have experience in open source engineering abd I can create pr and submit in a day
kindly assign this issue to me

[Cyberking99]

Hello,

I am a smart contract and full-stack developer interested in contributing to this issue. I am experienced with hands-on experience in Rust, Soroban, Go, Cairo, Solidity, and TypeScript.

I am passionate about building secure and scalable blockchain solutions, with a strong focus on writing clean, maintainable code. Based on the scope of this issue, I am confident I can deliver a complete solution within 24 hours of assignment.

I would appreciate the opportunity to contribute and help move this forward.