fix: update volunteer cancellation and requester cancellation

[berat-sayin]

Volunteer Cancellation:

• Volunteers now become unavailable when they cancel an assignment.
• The associated help request returns to PENDING status and is auto-assigned to another available
  volunteer.
• This is handled in backend/src/modules/availability/service.js within cancelMyAssignment.

Requester Cancellation:

• Requesters can now set their help request status to CANCELLED.
• This correctly triggers the removal of any active assignment, freeing the assigned volunteer.
• The volunteer remains available and can be immediately auto-assigned to another pending request.
• Changes were made to backend/src/modules/help-requests/validators.js, service.js, and repository.js, as
  well as backend/src/modules/availability/service.js and repository.js.