Add API payments module for capsule gift creation and status retrieval

[nottherealalanturing]

Description

The web app needs backend routes to create and inspect Stellar gift intents tied to capsules.

Scope

• Create payments controller/service in API
• Validate amount/asset/network rules
• Link payments to sender-owned capsules only
• Review key files: apps/api/src/modules/payments/**

Acceptance Criteria

• Authenticated sender can attach a gift intent to a draft or sealed capsule per defined rules
• Payment status is queryable from API
• Invalid ownership or invalid amount is rejected

Dependencies

Blocked by: Implement Stellar transaction service for creating gift funding intents

Stack

Express / Stellar