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>

fastlane/metadata/android/en-US/changelogs/4.txt

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+Live edge-detection overlay while aiming. Fully English UI. Public-ready release.

fastlane/metadata/android/en-US/changelogs/5.txt

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+Multi-language support: the interface is now available in English and Dutch, following your device language.

fastlane/metadata/android/en-US/changelogs/6.txt

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+Added German, French, Spanish, Italian and Portuguese translations (seven languages total).

fastlane/metadata/android/en-US/full_description.txt

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+PaperScan is a local, privacy-conscious document scanner. Scan paper documents with your camera, automatically detect the edges, correct the perspective, and bundle multiple pages into a single PDF. Share it through Android or upload it directly to your self-hosted Paperless-ngx.
+
+Everything happens on the device. Camera frames, edge detection (OpenCV) and PDF creation never leave your phone. The only network connection is the optional upload to the Paperless server you configure yourself. There is no AI, tracking or cloud processing.
+
+Features:
+
+* Multi-page capture with a live edge-detection overlay while aiming
+* Automatic perspective correction, with manual corner adjustment
+* Per-page filters: color, grayscale, black & white
+* Rotate, reorder and delete pages
+* Export to a multi-page PDF (A4 or original size)
+* Direct upload to Paperless-ngx, or share via the Android share sheet
+* Real upload feedback: Added, Duplicate or Failed
+* API token stored encrypted (Android Keystore)
+
+PaperScan requests only the Camera and Internet permissions. Internet is used solely to reach the Paperless server you configure.

fastlane/metadata/android/en-US/short_description.txt

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+Private, on-device document scanner for your self-hosted Paperless-ngx.

fastlane/metadata/android/en-US/title.txt

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+PaperScan