DOCS UPDATE: Fix Windows PowerShell keytool commands in BACKEND.md

### Is this related to an existing part of the documentation?

- [ ] Yes, it is related to an existing section

### What needs to be updated?

**Is there an existing issue for this?**
I have searched the existing issues and found none.

**##What happened?**
While setting up the backend on Windows, I found two
undocumented issues in BACKEND.md that completely block
Windows users from generating the SHA-1 fingerprint.

## Problem 1: %USERPROFILE% fails in PowerShell

The current keytool command uses %USERPROFILE% which is
a CMD environment variable. It does NOT expand in PowerShell.

Error received:
"Keystore file does not exist: %USERPROFILE%\.android\debug.keystore"

WRONG (current - fails in PowerShell):
keytool -list -v -keystore %USERPROFILE%\.android\debug.keystore

CORRECT (works in PowerShell):
keytool -list -v -keystore C:\Users\YOUR_USERNAME\.android\debug.keystore

## Problem 2: .android folder doesn't exist on fresh Windows machines

On Windows machines without Flutter or Android Studio installed,
the .android folder doesn't exist. This causes keystore
generation to fail with:
"FileNotFoundException: The system cannot find the path specified"

Fix: Add this step before the keytool command:
mkdir C:\Users\YOUR_USERNAME\.android

## Expected Behavior
Windows users should be able to generate SHA-1 fingerprint
without hitting undocumented errors.

## Environment
- OS: Windows 11
- Terminal: PowerShell
- Flutter: Not installed (backend-only contributor)

### Relevant Documentation Link (if any)

https://github.com/AOSSIE-Org/Ell-ena/blob/main/BACKEND.md

### Record

- [x] I agree to follow this project's Code of Conduct
- [x] I want to work on this update