Add repository transaction boundary patterns for multi-step persistence workflows

[David-patrick-chuks]

Summary

Define transaction boundary patterns so future multi-step persistence workflows can be implemented safely and consistently.

Context

As the backend grows into payments, wallets, and audit-sensitive operations, some flows will require multi-step database consistency guarantees.

Scope

• Define transaction boundary patterns for repositories/services
• Document where transactional coordination should live
• Keep the design modular and contributor-friendly
• Prepare for future multi-entity workflows

Acceptance Criteria

• The codebase has a clear pattern for transactional persistence workflows
• Repository and service responsibilities are documented more clearly
• The design supports future multi-step financial workflows
• Contributors can implement transaction-safe features more predictably

Out of Scope

• Full payment settlement implementation
• Cross-service distributed transactions

Dependencies

Depends on #7 and should align with #8.
Repository boundaries and real PostgreSQL-backed persistence should exist before transaction coordination patterns are finalized.