DOCS UPDATE: supabase secrets set --env-file ./supabase/functions/.env doesnot allow prefix SUPABASE_

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

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

### What needs to be updated?

## 📌 Current Documentation Issue

In Backend.md, the instructions say
```
supabase secrets set SUPABASE_SERVICE_ROLE_KEY=your-service-role-key
supabase secrets set SUPABASE_DB_URL=your-db-url
```
but supabase doesnot allow variables with prefix SUPABASE_

## ✨ Proposed Changes

- There is no need of SUPABASE_SERVICE_ROLE_KEY, as it is automatically injected by supabase
- Need to change SUPABASE_DB_URL to DATABASE_URL and update its usage across project.

## 📋 Additional Context

### Relevant Documentation Link (if any)

_No response_

### Record

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