[Expenditures] Add Create Expenditure Route (POST /expenditures)

[nourshoreibah]

User Story

As a user I want to record an expenditure so that spending is tracked accurately.

Description

• Add POST route to expenditures Lambda.
  Command:
  node tools/lambda-cli.js add-route expenditures POST /expenditures --body enteredBy:number,amount:number,description:string,projectID:number --status 201
• Insert into expenditures and link via proj_expend.
• Return inserted record.

Acceptance Criteria

• Returns 201 with expenditure data.
• Expenditure linked to correct project.