Add API Rate Limiting per User

[Mkalbani]

Description

Implement per-user rate limiting (not just IP-based).

Current State

• Global rate limit exists (10 req/min)
• Need user-specific limits

Acceptance Criteria

• Free users: 100 requests/hour
• Authenticated users: 1000 requests/hour
• Return proper 429 status with retry-after header
• Document in README

Tech Stack

• Use @nestjs/throttler
• Store limits in Redis (optional for MVP)

ETA: 6 hours

[Lynndabel]

Hi maintainer, please can you assign this issue to me? ETA: 6hours

[OmniZlatoon]

Hi maintainer sir, I'm called Junior Brown , a full-stack and smart contract engineer building on Stellar. I specialize in designing and shipping scalable, secure systems end-to-end, from Soroban smart contracts to production-ready backend infrastructure and user-facing applications.