Support SQL default expressions in datasource schema

[rmorehig]

Summary

• add defaultExpr(expression) to column validators for raw SQL defaults
• emit raw DEFAULT <expression> in generated datasource files when defaultExpr is used
• migrate parser/TS emitter now falls back to .defaultExpr(...) for non-literal datasource defaults (for example generateUUIDv4())
• add tests for validator modifiers, datasource generation, and migrate output
• document the new modifier in README

Verification

• pnpm exec vitest run src/schema/types.test.ts src/generator/datasource.test.ts src/cli/commands/migrate.test.ts
• pnpm typecheck