chore(dev): make first-run local validation reproducible

[SharanyoBanerjee]

Problem

Running pnpm lint in a fresh checkout fails before linting because dependencies are not installed locally:

sh: turbo: command not found
Local package.json exists, but node_modules missing, did you mean to install?

That is expected before pnpm install, but the repo also lacks a single documented "verify my setup" command and does not currently include migrations/seed scripts referenced by README.

Suggested fix

Add a clear first-run validation path:

• document pnpm install --frozen-lockfile
• add or document pnpm build, pnpm lint, and service-specific checks
• either add the referenced Prisma migrations/seed files or mark them planned
• consider a root check script once tests exist

Acceptance criteria

• A contributor can clone, install, and run one documented validation sequence.
• Setup docs do not reference missing seed/migration files as if they exist.
• The root scripts map cleanly to workspace scripts.