[BACKEND] Add database migration scripts instead of auto-create

[Trovic1]

Summary

Introduce versioned SQL migrations (and an npm run migrate script) rather than creating schema on boot.

Scope

• backend only

Likely files

• src/lib/db.js
• migrations/ (new)
• package.json (script)

Acceptance criteria

• Schema lives in numbered migration files
• A migrate command applies pending migrations idempotently
• App boot no longer silently creates tables (or does so only in dev)

Verification

• Run migrations on a fresh database and confirm schema matches

Labels: backend, enhancement, help wanted

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