feat(meta): per-share-link rate limiting via MemoryCache

[crrow]

Context

MetaService design (docs/design/meta-service.md lines 109-117) requires per-share-link rate limiting stored in MemoryCache (Redis), shared by Gateway and MetaService.

Acceptance Criteria

• Integrate Redis/Memcached for rate limit state
• Per-share-link rate limiting on GET /share/:token (resolve share)
• Configurable rate limit window and max requests
• Coordinate with Gateway's per-IP rate limiting

Dependencies

• Redis integration