Implement Stellar transaction service for creating gift funding intents

[nottherealalanturing]

Description

The current Stellar service only ensures testnet account funding. The MVP needs a gift-attachment flow for capsules.

Scope

• Define gift intent model and API contract
• Add Stellar service method to prepare funding instructions or transaction memo strategy
• Persist gift intent on capsule
• Review key files: apps/stellar/src/services/**, API payments module

Acceptance Criteria

• Sender can initiate a gift intent for a capsule
• Gift amount, asset, and status are persisted
• Service supports Stellar testnet only for now

Dependencies

Blocked by: Phase 1 capsule model/service issues
Blocked by: Phase 1 Stellar bootstrap fix

Stack

Stellar / Express