refactor: decompose idempotency and subscription orchestration

[gabedalmolin]

Objective

Reduce structural risk in the application layer by decomposing idempotency and subscription orchestration hotspots into smaller cohesive modules while preserving public behaviour.

Scope

• decompose packages/application/src/idempotency.ts into clearer units of responsibility
• decompose packages/application/src/subscription.ts incrementally, following the same stable-facade approach used in invoice orchestration
• keep public application-layer contracts stable unless an intentional change is explicitly documented
• improve navigability, testability, and change safety in the application core
• avoid mixing new billing features into this refactor

Acceptance Criteria

• idempotency orchestration is split into smaller, cohesive modules with clearer boundaries
• subscription orchestration is materially easier to navigate and no longer concentrated in a single large hotspot
• public surface remains stable or any intentional change is explicitly documented
• existing behaviour is preserved through focused validation and test coverage
• npm run quality:gate passes
• DATABASE_URL='postgresql://grantledger_app:grantledger_app@localhost:5432/grantledger_rls' npm run test:pg passes