[TASK] [Backend] Add MongoDB to docker-compose.yml and update .env config

[oliviaa28]

Description

Add MongoDB service to the existing docker-compose.yml to allow local development without depending on the Atlas connection string. Update environment variable configuration to remove hardcoded credentials.

Scope

• Add MongoDB service to docker-compose.yml
• Add MONGO_URI to .env.example
• Update application.properties to use ${MONGO_URI} instead of hardcoded connection string

Done When

• MongoDB service starts successfully with docker-compose up -d
• application.properties reads MONGO_URI from .env
• .env.example contains MONGO_URI so other developers know what to configure
• No hardcoded credentials exist in any committed file