Volunteer — Adopt Domain Exceptions & Unified Error Handling

## Description
Refactor **volunteer** module to the new exception model.

## Scope
- Controllers: `src/modules/volunteer/presentation/controllers/*`
- Services/use-cases: `src/modules/volunteer/*`

## Tests
- Validate list/create/update/delete errors mapped correctly.

## Acceptance criteria
- Only domain exceptions used; tests green.
