chore(dx): root Makefile

[pentaxis93]

Summary

Create a Makefile at project root with targets for common operations, reducing the need to remember directory-specific commands. Many exemplary projects (Open WebUI, full-stack-fastapi-template) provide this convenience. The Makefile should surface the commands already documented in CLAUDE.md.

Part of: #3

Scope

• Makefile — create at project root with targets for: server-dev, server-test, server-lint, server-install, client-run, client-lint, lint (both), test (both), db-migrate, format, help (default target listing all available targets)

Acceptance criteria

• Makefile exists at project root
• make help (or bare make) lists all available targets with descriptions
• Server targets work: make server-test, make server-lint, make server-dev
• Client targets work: make client-lint
• Aggregate targets work: make lint, make test
• Database target works: make db-migrate
• All targets use the same commands as documented in CLAUDE.md quick reference

Dependencies

None — Layer 0, no blockers.