Add F-Droid publishing guide and self-hosted repo helper script

docs/FDROID.md covers both a self-hosted F-Droid repo and the official
f-droid.org route (incl. the prebuilt-OpenCV caveat). scripts/update-fdroid-repo.sh
pulls the release APKs from Gitea and rebuilds the signed index.

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

scripts/update-fdroid-repo.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+#
+# Rebuilds a self-hosted F-Droid repo for PaperScan: downloads every release APK
+# from the Gitea repository and regenerates the (signed) index.
+#
+# Prerequisites:
+#   * fdroidserver installed (`pipx install fdroidserver`)
+#   * `fdroid init` run once in $FDROID_HOME (creates config.yml + keystore)
+#   * JDK + Android SDK build-tools on PATH
+#
+# Usage:
+#   FDROID_HOME=~/paperscan-fdroid ./scripts/update-fdroid-repo.sh
+#
+set -euo pipefail
+
+FDROID_HOME="${FDROID_HOME:-$HOME/paperscan-fdroid}"
+GITEA_API="${GITEA_API:-https://git.geyskens.eu/api/v1/repos/sam/paperscan}"
+
+if [ ! -f "$FDROID_HOME/config.yml" ]; then
+  echo "No F-Droid repo at $FDROID_HOME. Run 'fdroid init' there first (see docs/FDROID.md)." >&2
+  exit 1
+fi
+
+mkdir -p "$FDROID_HOME/repo"
+cd "$FDROID_HOME"
+
+echo "Fetching release APK list from Gitea…"
+curl -fsSL "$GITEA_API/releases?limit=50" \
+  | python3 -c "import sys,json;[print(a['browser_download_url']) for r in json.load(sys.stdin) for a in r.get('assets',[]) if a['name'].endswith('.apk')]" \
+  | while read -r url; do
+      dest="repo/$(basename "$url")"
+      if [ -f "$dest" ]; then
+        echo "  have $(basename "$url")"
+      else
+        echo "  get  $(basename "$url")"
+        curl -fsSL "$url" -o "$dest"
+      fi
+    done
+
+echo "Rebuilding index…"
+fdroid update --create-metadata --pretty
+
+echo
+echo "Done. Publish the repo, e.g.:"
+echo "  rsync -a --delete \"$FDROID_HOME/repo/\" web:/var/www/fdroid/repo/"