DPS Convert

Internal document conversion. The in-house replacement for CloudConvert — same conversions, no metered third-party account to run out of credits.

`POST /api/v1/html-to-pdf`	Chrome 137 — replaces CloudConvert engine "chrome"
`POST /api/v1/pdf-split`	pdf-lib — replaces "pdf/split-pages" (qpdf) + export/s3
`POST /api/v1/pdf-to-jpeg`	MuPDF @ 300 DPI — replaces engine "mupdf"
`POST /api/v1/tif-to-jpeg`	sharp — replaces engine "imagemagick"
`POST /api/v1/png-to-jpeg`	sharp — replaces engine "imagemagick"

All endpoints require Authorization: Bearer $CONVERT_AUTH_TOKEN. Payloads over ~3 MB must travel by S3 reference — Vercel caps request and response bodies at 4.5 MB.