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>

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

@@ -0,0 +1,19 @@
+Calendarr is the Android client for your own self-hosted Calendarr server — a private, ad-free calendar whose data stays on your infrastructure.
+
+Bring together the sources you already use:
+* Local calendars on the server
+* CalDAV accounts
+* iCal / ICS subscriptions
+* Google Calendar
+* Home Assistant
+
+Features:
+* Month, week, day, quarter and agenda views
+* A left drawer to show/hide and reorder your calendars, switch groups, and reach the menu
+* Share calendars with other users and organise them in groups
+* Birthdays: a dedicated birthday calendar with age and reminders, optionally imported from your contacts
+* Per-setting cross-device sync — choose exactly which settings follow you across web, iOS and Android
+* Fully customisable colours and a dark interface
+* Event reminders / notifications
+
+Calendarr requires access to a Calendarr server (self-hosted). Enter your server URL, then sign in.