Groups the first-page BufferedImage and the source's total page count
into a SourcePreview record so both values travel through generate()
together. PDFs get pdf.getNumberOfPages(); image uploads always get 1
(a scan is one page from the user's perspective). The page badge on
the thumbnail row uses this value to show "1 / N" for multi-page
letters without a separate round-trip.
Refs #305
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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