Add a Makefile for common developer commands

[sanmaxdev]

Background

Common tasks (start the stack, run migrations, lint, test) are spread across the README. A Makefile with simple targets would lower the barrier for new contributors.

Tasks

• Add targets such as make up, make down, make migrate, make lint, make test
• Cover both backend and frontend where it makes sense
• Reference the targets in CONTRIBUTING.md

Acceptance criteria

• Targets work on a clean checkout
• Documented in CONTRIBUTING

Good first issue, comment to claim it. See CONTRIBUTING.md.