* Security: validate remote OCR endpoint against internal SSRF
Adds PAPERLESS_REMOTE_OCR_ALLOW_INTERNAL_ENDPOINTS (default true)
and validates remote_ocr_endpoint via validate_outbound_http_url
on the config serializer, matching the existing LLM endpoint handling.
* Validates te outbound url again right before use
* cover empty-value branch of validate_remote_ocr_endpoint because coverage
* re-validate remote OCR endpoint on every outbound request