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

[brijesh-amin]

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.