Add --ghostscript-jpeg-quality and --ghostscript-jpeg-maxdpi

Expose Ghostscript's -dJPEGQ and image downsampling switches as
advanced, plugin-scoped options for tuning PDF/A output, without
polluting the central OcrOptions registry. The optimizer's existing
--jpeg-quality remains the recommended JPEG quality control.

- GhostscriptOptions gains jpeg_quality and jpeg_maxdpi fields and CLI
  args (advanced help text). jpeg_quality=0 is honored as Ghostscript's
  maximum compression rather than being silently coerced to the default.
- _exec.ghostscript.generate_pdfa() forwards both values; when
  jpeg_maxdpi is set, downsample threshold is pinned at 1.0.
- _get_plugin_options falls back to extra_attrs for namespaced fields
  so plugins can own their options without registering them centrally.
- Documentation explains the rationale: Ghostscript is the legacy path
  (pypdfium + verapdf is preferred in v17+), the optimizer is the
  supported file-size lever, and lowering quality is almost always a
  better trade than downsampling.

docs/optimizer.md

@@ -98,7 +98,16 @@ If `pngquant` is installed, OCRmyPDF will use it to perform quantize
 paletted images to reduce their size.
 
 The quality of JPEGs may be lowered, on the assumption that a lower
-quality image may be suitable for storage after OCR.
+quality image may be suitable for storage after OCR. Use `--jpeg-quality`
+to control the optimizer's JPEG quality target. The optimizer is the
+recommended way to reduce JPEG image sizes: it applies consistently
+regardless of whether Ghostscript was used to produce a PDF/A.
+
+If you specifically need to tune Ghostscript's own PDF/A image handling
+(for example, to force a hard DPI cap), see
+[Advanced Ghostscript tuning](advanced.md#advanced-ghostscript-tuning)
+for the separate `--ghostscript-jpeg-quality` and
+`--ghostscript-jpeg-maxdpi` options.
 
 It is not possible to optimize all image types. Uncommon image types may
 be skipped by the optimizer.