…snapshot

Fixes langflow-ai#1375

Previously, DOCLING_SERVICE_URL and HOST_IP were resolved once at module
import time in src/api/docling.py. Any change to DOCLING_SERVE_URL after
onboarding (e.g. pointing to a different docling-serve instance) was silently
ignored until the backend process was fully restarted.

Changes:
- src/api/docling.py: replace the module-level static URL assignment with a
  get_docling_service_url() function that reads os.getenv("DOCLING_SERVE_URL")
  on every call, falling back to the auto-detected host when not set.
  health() now calls get_docling_service_url() per request.
- src/utils/docling_client.py: import get_docling_service_url() instead of
  the stale DOCLING_SERVICE_URL string so every conversion request resolves
  the current URL from the environment at call time.