M11: the batch optimize step's ppi reaches black-and-white images

The batch step took Reduce File Size's monochrome settings unchanged, so
`ppi: 600` (or `--ppi 600`) kept colour and grey images at 600 ppi but
halved 600 ppi black-and-white scans to 300. Black-and-white images now
go to the larger of ppi and 300 ppi: a higher ppi keeps their
resolution, and a lower one doesn't take scanned text below 300. The
doc_batch description and docs/cli-batch.md say so.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/cli-batch.md

@@ -27,9 +27,11 @@ new engine importers become available here automatically. Without `convert`,
 only `.pdf` inputs are processed. Unsupported extensions are reported as skipped.
 
 - `optimize` uses Reduce File Size settings: JPEG quality 60, resampling to 150 ppi
-  above 225 ppi (black-and-white images stored as CCITT Group 4 when smaller), plus the
-  existing document cleanup. `--ppi` (9–1600) sets the target
-  and a threshold of 1.5 times that value; `--quality` (1–100) sets JPEG quality.
+  above 225 ppi (black-and-white images to 300 ppi above 450 ppi, stored as CCITT
+  Group 4 when smaller), plus the existing document cleanup. `--ppi` (9–1600) sets the
+  target and a threshold of 1.5 times that value; black-and-white images go to the
+  larger of `--ppi` and 300 ppi, so a lower value keeps scanned text legible and a
+  higher one keeps their resolution. `--quality` (1–100) sets JPEG quality.
   These settings can lose image detail. A candidate that is not smaller retains
   the bytes entering that step. This is not an images-only or lossless preset.
 - `ocr` uses the installed engine and its supported languages. `--dpi` (72–600,