feat: add price alert webhook registration for creator key price thresholds

[ogazboiz]

Summary

Adds a price-alert system so fans can register a target price and a callback URL, and the server fires the webhook when a creator's key price crosses that threshold. Alerts are one-shot: they fire once when the threshold is crossed, then are removed.

This mirrors the conventions of the recently merged trade-webhook feature (src/modules/webhooks/): zod validation, controller/router/service split, Prisma access via src/utils/prisma.utils.ts, and the same delivery/retry/backoff approach driven by WEBHOOK_RETRY_MAX_ATTEMPTS / WEBHOOK_RETRY_BASE_DELAY_MS.

Changes

• POST /api/v1/alerts registers an alert { creator_id, wallet_address, target_price, direction, callback_url } (direction ∈ above | below) and returns a unique alert ID.
• DELETE /api/v1/alerts/:id cancels a pending alert before it fires.
• Price-cross evaluation is wired into the trade-event processing path. A new processTradeEvent orchestrator (src/utils/trade-event.utils.ts) fans a single trade event out to both webhook dispatch and alert evaluation, so alerts are not a parallel pipeline.
  • above fires when the new price rises to/past the target; below fires when it drops to/past the target. Opposite movement does not fire.
• Trigger payload: { creator_id, triggered_price, target_price, direction, timestamp }.
• Successful delivery deletes the alert (one-shot). Failed delivery is retried with exponential backoff up to the configured max attempts, after which the alert is marked FAILED.
• New Alert Prisma model (prisma/schema/alert.prisma) + migration (20260619000000_add_price_alerts).

Test plan

• pnpm build — passes.
• pnpm lint — passes.
• pnpm test -- src/modules/alerts — 17 tests pass (service unit tests + integration tests).
• Integration tests cover: above trigger, below trigger, cancellation, and failed-delivery retry (Prisma + fetch mocked, following the suite's existing mocking approach so no live DB is required).
• Pre-existing DB-dependent suites (and one unrelated baseline async-logging test) fail locally only because no Postgres is available; they are unchanged by this PR.

Closes #423