Reorganize `accounts` app under `backend/accounts/`

[pashiav]

Description:
Starting the backend folder reorganization by moving the accounts app into a new backend/ directory under the specifyweb/ root folder.

Intentionally not using a src/ prefix (like src/backend/) since the existing frontend/ folder is already located at the root level to keep consistency.

Notes:

• This is the 1st folder that is moving as part of the overall reorganization plan.
• This PR will focus only on moving and reorganizing the accounts app. No logic or functional changes will be made.