feat(async): Add django-q2 with ORM broker and demo job

[taylor-osler-sentry]

Summary

• Adds django-q2 as a dependency and configures it with the ORM broker (backed by the existing PostgreSQL default database)
• Introduces firetower.incidents.tasks.schedule_demo as the first queue task (stub that prints "hello world"), registered via a data migration
• Extends docker/entrypoint.sh with a worker command that starts qcluster, and refactors all branches to use exec for proper signal handling in Docker

Test plan

• Run python manage.py migrate — confirm 0015_schedule_demo applies cleanly and a schedule_slo_nag row appears in django_q_schedule
• Run python manage.py qcluster — confirm the worker starts and picks up the scheduled task
• Run docker/entrypoint.sh worker — confirm the container starts the qcluster process
• Unapply migration (migrate incidents 0014) — confirm the schedule row is removed

🤖 Generated with Claude Code

[linear-code]

RELENG-538 Stand up some kind of async task running capability for Firetower