Docling and local Crawl4AI are now opt-in, installed on first container startup
via OPEN_NOTEBOOK_ENABLE_DOCLING / OPEN_NOTEBOOK_ENABLE_CRAWL4AI, keeping the
default image lean (Crawl4AI un-bundled). Downloads cache on the /app/data
volume; installs are blocking with loud logs and degrade-don't-die on failure.
A GET /api/capabilities probe reports actual availability and the Settings UI
gates the Docling/Crawl4AI engines and OCR toggle (with env-var hints) until the
runtime is present. Docs folded from #1121 and rewritten to opt-in; ADR-007 added.
Closes#1122. Closes#1105.
