Get the repo ready for F-Droid

- License: AGPL-3.0-or-later (LICENSE, every package.json, README).
- MUSICSHARE_FOSS=1 builds leave out the only closed-source code, Google's
  ML Kit / Play Services barcode scanner bundled by expo-camera; the scan
  screen then decodes small camera snapshots with jsQR instead.
- Expo modules build from source rather than from precompiled AARs, as
  F-Droid requires (that's also what makes the scanner switch take effect).
  Verified: a FOSS build contains no ML Kit or Play Services classes.
- Release builds default to the production server, and come out unsigned
  without our upload key (F-Droid signs with its own). versionCode 1.
- Store listing in fastlane/metadata/android, a draft fdroiddata recipe in
  fdroid/, and docs/FDROID.md with the submission steps.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MzFywujuH9TiuXbr9vjoX8

README.md

@@ -149,3 +149,13 @@ Links are https universal links, so on phones they open the native app. Missing
 3. Android media-session reader; Apple Music via MusicKit; RevenueCat billing
 4. Listening history, reactions/comments on tracks, shareable profile cards (server-rendered OG images)
 5. Ops: deploy API/worker containers (Fly.io, Railway or ECS), managed Postgres/Redis, Sentry, metrics on poll latency and per-provider error rates
+
+## F-Droid
+
+The Android app builds with no closed-source code when `MUSICSHARE_FOSS=1` is set. See
+[docs/FDROID.md](docs/FDROID.md) for the build, the store listing in `fastlane/`, and how to submit.
+
+## License
+
+musicshare is free software under the [GNU Affero General Public License v3.0 or later](LICENSE).
+If you run a modified version as a service for others, you must make your changes available to them.