Create User Data Export

Kick off a GDPR Subject Access Request (SAR) export for the caller.

Enqueues a background worker that zips the caller's personal data into the user-exports S3 bucket. One non-expired export per user per 24h; a request inside that window returns 429 with Retry-After set from the existing job.

Args: auth: Authenticated user context.

Returns: 202 Accepted with the job id and initial pending status.

Raises: HTTPException: 429 inside the 24h rate-limit window; 500 on failure.