#523 - Extract plan vertical slice to controller/service/repository boundary

Parent: #523

## Objective
Create a modern boundary for one high-value plan flow by separating transport concerns from business logic and data access.

## In scope
- Select one plan workflow currently implemented in `PlanController`
- Introduce service module(s) under `src/services/plan/` for orchestration/business rules
- Introduce repository abstraction(s) over db2 access for the selected flow
- Keep controller as thin request/response adapter
- Add/adjust tests to protect behavior

## Acceptance criteria
- Controller delegates core logic to service
- Service does not depend on Express req/res
- Repository encapsulates query implementation for the selected flow
- Existing endpoint behavior remains compatible
- Regression tests cover service behavior and endpoint contract

## Notes
Use this as the reference pattern for later controller decompositions.