P0: Add idempotency to paid checkout fulfillment

[devcodex2025]

Source: docs/GROWTH_AND_AGENT_BACKLOG.md

Area: Payments / credits
Priority: P0
Owner type: agent or human contributor
Files likely affected: src/services/payment-service.js, src/services/user-service.js, schema/migrations

Goal:
Prevent duplicate credits when payment callbacks or checkout confirmations repeat.

Acceptance criteria:

• Implementation matches the backlog goal.
• Tests are added or a clear reason is documented if tests are not applicable.
• User-facing errors or recommendations are actionable and do not leak secrets.
• Documentation/API notes are updated when behavior changes.

Risk:
Medium. Keep the change scoped and avoid unrelated refactors.