extractPDFCover previously only saved embedded raster images from page 1,
so vector/text-first-page PDFs (e.g. InDesign exports like Data Structures
the Fun Way) ended up with no cover and a dashboard placeholder. When no
embedded image is found it now falls back to rendering page 1 with
pdftoppm (poppler-utils), saving the same {pdf}.cover.jpg sidecar.
Also adds MediaScanner.RescanMediaItem, which re-extracts metadata for a
single media item (resolving its on-disk path from library folders) so
previously imported books can backfill covers without a full force rescan.
Dockerfile installs poppler-utils in the final and test-runner stages.
john-okeefe
0614795ca2