feat(cli): add --json option for machine-readable output

Add --json flag to prepare-db command for machine-readable output:

- Success output includes: mode, action, database, monitoringUser,
  applied steps, skippedOptional, and warnings
- Error output includes: mode, error object with message, step,
  code, detail, hint, and httpStatus (for Supabase)
- Verify output includes: verified status, missingRequired/Optional
- Works for both Supabase and direct PostgreSQL modes

Example success:
{
  "success": true,
  "mode": "supabase",
  "action": "apply",
  "database": "postgres",
  "monitoringUser": "postgres_ai_mon",
  "applied": ["01.role", "02.permissions", ...],
  "skippedOptional": ["03.optional_rds"],
  "warnings": []
}

Example error:
{
  "success": false,
  "mode": "supabase",
  "error": {
    "message": "Failed at step \"01.role\": permission denied",
    "step": "01.role",
    "code": "42501"
  }
}

Author: claude