docs(readme): note Ghostscript dependency for full PDF/A-2b conformance

L2 from logical-beaming-brooks.md plan: the format table advertises
PDF/A-2b output, but the README never mentioned that full veraPDF
conformance requires Ghostscript. Self-hosters running the local-Python
install path could end up with markup-only PDF/A output that fails
veraPDF when the source has unembedded fonts.

Footnote on the PDF row + a one-paragraph note under the format table:
  - Docker image bundles Ghostscript (no action needed)
  - Local-Python installs follow docs/installation.md
  - Markup-only fallback is the visible behaviour without it

No code change. The fallback path in app/converters/pdfa.py already
exists and structured-logs `mode=markup` vs `mode=rerender` so operators
can spot the gap; this just makes the dependency documented.

Author: MrChengLen

README.md

@@ -83,7 +83,7 @@ and [`COMMERCIAL-LICENSE.md`](COMMERCIAL-LICENSE.md).
 |---------------|------------------------------------------------------------------|-----------------------------------------------|
 | **Images**    | HEIC, HEIF, JPG, JPEG, PNG, WebP, BMP, TIFF, GIF, ICO           | JPG, PNG, WebP, BMP, TIFF, GIF, ICO           |
 | **Documents** | DOCX, TXT, Markdown (`.md`)                                      | PDF, TXT, HTML                                |
-| **PDF**       | PDF                                                              | TXT, PDF/A-2b                                 |
+| **PDF**       | PDF                                                              | TXT, PDF/A-2b<sup>†</sup>                     |
 | **Spreadsheets** | XLSX, CSV, JSON                                               | CSV, XLSX, JSON                               |
 | **Audio**     | MP3, WAV, FLAC, OGG, M4A, AAC, WMA, Opus                        | MP3, WAV, FLAC, OGG, M4A, AAC, WMA, Opus     |
 | **Video**     | MP4, MOV, AVI, MKV, WebM, FLV, WMV                              | MP4, MOV, AVI, MKV, WebM, FLV, WMV           |
@@ -92,6 +92,14 @@ and [`COMMERCIAL-LICENSE.md`](COMMERCIAL-LICENSE.md).
 Images: JPG, PNG, WebP, TIFF · Video: MP4, MOV, AVI, MKV, WebM
 Compress mode supports both *by quality %* and *by target size MB*.
 
+<sup>†</sup> Full PDF/A-2b conformance (passes
+[veraPDF](https://verapdf.org/) validation) requires
+[Ghostscript](https://www.ghostscript.com/) on the host. The Docker
+image bundles it; for local-Python installs see
+[`docs/installation.md`](docs/installation.md). Without Ghostscript,
+`pdf → pdfa` falls back to a markup-only output that veraPDF will
+reject if the source PDF has unembedded fonts.
+
 ---
 
 ## Quickstart