[GSSoC26] chore: Update docker-compose.yml - remove deprecated version, add admin service

[ashrion]

Description

This PR updates docker-compose.yml to remove the deprecated version key and add the admin service.

Changes

• Removed deprecated version: "3.8" key (deprecated in Docker Compose v2+)
• Added admin service with proper dependencies and networking
• Added explicit network configuration for service communication
• Added MongoDB data volume for persistence

Motivation

• The version key is deprecated and shows warnings in modern Docker Compose
• Admin service was missing from docker-compose, making full-stack deployment incomplete
• Explicit networking improves service discovery and communication

Testing

• Run docker-compose up --build and verify all three services start
• Verify frontend, backend, and admin are accessible
• Verify services can communicate via service names

Closes #444

GSSoC26 contribution