Add GPL-3.0 license and F-Droid/Fastlane store metadata

- LICENSE: GNU GPL v3.0 (verbatim).
- README: License section.
- fastlane/metadata/android/{en-US,nl}: title, short/full description, changelogs
  for versionCodes 4-6. Used by both a self-hosted F-Droid repo and f-droid.org.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -130,3 +130,12 @@ Tracked as issues in the repository:
 
 > OCR is intentionally **not** done on-device: Paperless-ngx already runs OCR (via
 > OCRmyPDF/Tesseract) when it consumes the uploaded document.
+
+## License
+
+PaperScan is free software licensed under the **GNU General Public License v3.0** — see
+[LICENSE](LICENSE). You may use, study, share and modify it; derivative works must stay
+under the GPL.
+
+Store listing metadata (title, descriptions, changelogs) lives under
+`fastlane/metadata/android/`, in the F-Droid/Fastlane layout.