fix: use NestJS HTTP exceptions in vouching service

[EmeditWeb]

Problem

The vouching service (lines 51, 78, 147, 165, 184) throws new Error(...) instead of NestJS HTTP exceptions like new InternalServerErrorException(...). These will result in unformatted 500 responses with no structured error code.

User impact: API clients receive inconsistent error formats (plain text 500 vs structured JSON).

Root Cause

Error handling was not updated to use NestJS patterns.

What To Build

1. Replace all new Error(...) with appropriate NestJS HTTP exceptions
2. Use NotFoundException, BadRequestException, InternalServerErrorException as appropriate
3. Verify same issue in vendors.service.ts (line 63)

Files To Touch

• src/modules/vouching/vouching.service.ts — fix error handling
• src/modules/vendors/vendors.service.ts — line 63 fix error handling

Acceptance Criteria

• All errors use NestJS HTTP exceptions
• API returns structured JSON error responses
• Build passes

Mandatory Checks

• context/ files read
• npm run build passes
• PR references this issue

[Doiae9]

I would work on this issue