[Donors] Add Get All Donors Route (GET /donors)

## User Story
As a user I want to **see all donor organizations** so that I can track who contributes to which project.

## Description
- Add GET route to `donors` Lambda.  
  Command:  
  `node tools/lambda-cli.js add-route donors GET /donors`
- Query all donors from the `donors` table.

## Acceptance Criteria
- Returns 200 and list of donors.
- Returns empty array if none exist.