Add django-linear-migrations for linear migration history

[valentijnscholten]

Summary

This PR adds django-linear-migrations to DefectDojo to maintain a linear migration history and prevent merge migration conflicts.

This will provide two benefits:

• Prevent accidental merging of PRs with multiple leaf migrations or conflicting migrations
• Provide an easy manage rebase-migration command to fix it locally and push the branch/PR. --> UPDATE: while testing this I realized this is quite complicated and manual resolution is much easier.

What Changed

• Updated settings.dist.py to conditionally load the package when DEBUG=True (development only)
• Updated docker-compose.override.dev.yml to use target: development for all containers
• Added comprehensive documentation in CONTRIBUTING.md for resolving migration conflicts
• Added documentation in AGENTS.md for AI agents
• Created dojo/db_migrations/max_migration.txt tracking file

How It Works

• Transparent: Developers continue using makemigrations as normal - it automatically updates max_migration.txt
• Development only: Only loaded when DEBUG=True, following the same pattern as django-debug-toolbar
• Conflict resolution: When migration conflicts occur, use python manage.py rebase_migration dojo to automatically fix them

[mtesauro]

Approved