Implement creator vesting schedule endpointsfeat(vesting): add vesting schedule endpoints for creator token release tracking

[Daveeee-tech]

closes #909
Summary

Adds API endpoints exposing vesting schedule data for creator key allocations, reading from Soroban on-chain state and caching for fast responses.

Scope
GET /keys/:id/vesting — returns vesting config, cliff, duration, and current claimable amount.
Vesting data is fetched from the Soroban contract and cached with a 60s TTL to keep response times low.
Cache is invalidated on the VestingClaimed contract event, so claims are reflected promptly instead of waiting out the TTL.
Vesting endpoints are restricted to creator-only access, enforced via JWT.
Adds a claimed-history endpoint returning past claim transactions (with tx hashes) for a given key.
Implementation Notes
Cache invalidation is event-driven (listening for VestingClaimed) rather than purely TTL-based, to avoid serving stale claimable amounts right after a claim.
Creator-only authorization is enforced at the endpoint/middleware level via JWT, not left to the client.
Testing
Unit tests for cliff/duration/claimable-amount calculation against known vesting configs.
Integration test confirming cache is served within TTL and invalidated correctly on a VestingClaimed event.
Auth tests confirming non-creator JWTs are rejected on vesting and claimed-history endpoints.
Claimed-history test confirming returned transactions match on-chain claim events, including tx hashes.
Load/latency check confirming cached responses stay under 200ms.

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