DOCS UPDATE: Incorrect Flask command path in backend installation guide

### 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 the [Installation Guide → Quick Start ](https://github.com/AOSSIE-Org/Devr.AI/blob/main/docs/INSTALL_GUIDE.md#quick-start)→ Step 8 (Start the backend server), the documentation instructs users to run the following command from the backend directory:

`flask --app api/index.py run --debug --port 5000
`

However, this command fails when executed from the backend/ directory because the Flask application file (index.py) is not located at backend/api/index.py. The actual Flask app is located at:

`backend/app/database/falkor/code-graph-backend/api/index.py
`

As a result, running the command exactly as written leads to the error:

`Error: Could not import 'index'
`

This can be confusing for new contributors, especially on Windows.

## ✨ Proposed Changes

**Update Step 8 of the installation guide to reflect the correct working directory and Flask app path.**

Suggested corrected instructions:

```
# Terminal 3 (Start graphDB / Code Graph Backend)
cd app/database/falkor/code-graph-backend
flask --app api.index run --debug --port 5000
```

Optionally, the guide could also mention that:
The Flask command must be run from the code-graph-backend directory

This change would make the setup instructions accurate and reduce onboarding friction.

## 📋 Additional Context

This issue was encountered during a fresh local setup following the installation guide step-by-step.
The rest of the backend (FastAPI, Docker services, Discord bot) works as expected once this path issue is resolved.

### Relevant Documentation Link (if any)

https://github.com/AOSSIE-Org/Devr.AI/blob/main/docs/INSTALL_GUIDE.md#quick-start

### Record

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