Service: Rewrite template CRUD for linked list step model

[b-at-neu]

Summary

Rewrite all ProcessStep service functions to work with the previousStepId linked list model instead of order Int. Supersedes #595 and #596.

Functions to Rewrite

getProcessTemplateAsSequence(id)

• Fetch all steps for template
• Resolve linked list into ProcessStepSequence[] tree starting from root steps (parentStepId=null, previousStepId=null)
• For each step, collect its branches (steps with parentStepId=step.id) as separate sequences

addProcessStep(templateId, data)

• Append to root sequence: find last step in root (step with parentStepId=null where no other step has previousStepId=that.id), set new step's previousStepId to it
• Support optional afterStepId to insert mid-sequence

addBranchStep(templateId, parentStepId, data)

• Start a new branch off parentStepId
• Find last step in existing branches for that parent, or create first step with previousStepId=null
• Each distinct branch is a separate linked sequence

deleteProcessStep(stepId, templateId)

• Re-link: set previousStepId of step's successor to step's previousStepId
• Soft-delete the step

moveProcessStep(templateId, stepId, direction)

• Swap previousStepId links with the adjacent step in the same sequence

Purchase queries

• Update getPurchaseProcess and getBatchPurchaseProcessData to include previousStepId

Blocked By

Blocked by #611 (schema) and #612 (types)

Supersedes #595, #596

Part of #530

[b-at-neu]

Blocked by #611 and #612