Docker/Infrastructure: Add Mailpit/Any other SMTP service to docker-compose

[preetkaran20]

Purely infra, no code changes. Touches docker-compose.yml and docker-compose.local.yml only in multiple repos from github.com/SasanLabs/VulnerableApp and github.com/SasanLabs/VulnerableApp-facade and github.com/SasanLabs/LLMForge

Add mailpit or Any other SMTP service (axllent/mailpit:latest)
Expose port 8025 externally, keep 1025 internal via expose
Add mailpit_data volume
Add mailpit to VulnerableApp-facade depends_on
Mirror the block in docker-compose.local.yml
Update README with Mailpit UI URL

Read more about design from issue: #648

[preetkaran20]

#648 parent issue

https://github.com/SasanLabs/VulnerableApp/wiki/Design-Document:-SMTP-Server-Integration-for-VulnerableApp Design doc