feat(cli): add shared PDF-to-image rendering and document usage

add shared render_pdf_pages helper for consistent rasterization
reuse the same renderer in regression test and CLI flow
add new render-image CLI command to export PDF pages as PNG
support single-page render (--page) and full export (--all-pages)
add configurable output path and DPI options
document the new rendering command and options in README

config.json

@@ -30,5 +30,6 @@
     "pdf_field_erstellungsdatum": "unterschrift.datum",
     "pdf_pauschale_prefix": "pauschale_5",
     "pauschale_present_value": "1",
-    "pdf_use_display_values": true
+    "pdf_use_display_values": true,
+    "pdf_need_appearances": false
 }