Answer extracted PDFs from the document cache: fresh for PDFTTL, then a
conditional GET (304 reuses the stored text, a 200 is extracted and stored
without a second GET). Store after extraction with the response hash, ETag,
fetch time and extracted pages. Route pages/grep through pdfdoc.Options with
400s for bad ranges and non-PDFs, and refuse a question when the document
exceeds PDFMaxQuestionChunks with a pointer to grep and pages.
Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>