chore: GPLv3 license + F-Droid packaging (metadata, recipe, guide)

- LICENSE: GNU GPLv3
- fastlane/metadata (de-DE + en-US): title, descriptions, changelog, icon —
  used by both the own F-Droid repo and the official listing
- fdroid/com.scarriffle.calendarr.yml: ready-to-submit fdroiddata build recipe
- FDROID.md: how to run the own repo (fdroid.scarriffle.com) and submit to
  official F-Droid, incl. the signing-key caveat

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

fdroid/com.scarriffle.calendarr.yml

@@ -0,0 +1,30 @@
+# F-Droid build recipe for the OFFICIAL repository.
+# Submit this file to https://gitlab.com/fdroid/fdroiddata as
+#   metadata/com.scarriffle.calendarr.yml
+# (see FDROID.md). F-Droid clones the source below, builds it on their servers
+# and signs it with F-Droid's key. Requires the SourceCode repo to be publicly
+# clonable and a git tag `v<versionName>` (e.g. v1.1.0) for each release.
+
+Categories:
+  - Time
+License: GPL-3.0-or-later
+AuthorName: Scarriffle
+WebSite: https://calendar.scarriffle.com
+SourceCode: https://git.scarriffle.com/Scarriffle/Calendarr-Android
+IssueTracker: https://git.scarriffle.com/Scarriffle/Calendarr-Android/issues
+
+RepoType: git
+Repo: https://git.scarriffle.com/Scarriffle/Calendarr-Android.git
+
+Builds:
+  - versionName: 1.1.0
+    versionCode: 2
+    commit: v1.1.0
+    subdir: app
+    gradle:
+      - yes
+
+AutoUpdateMode: Version v%v
+UpdateCheckMode: Tags
+CurrentVersion: 1.1.0
+CurrentVersionCode: 2