fix(android): publish releases to Open testing track (not internal)

[TaprootFreak]

Problem

release_track in android/fastlane/Fastfile was hard-pinned to internal, so every release only ever reached the Internal testing track. The Open testing track therefore never received new builds — it stayed stuck on an old version (1.0.84), and public opt-in testers (via play.google.com/apps/testing/swiss.realunit.app) got "not found" when trying to install.

Fix

Switch the binary upload track to beta (Open testing) so each release lands in the public, link-shareable channel.

Notes / to verify

• Track name: beta is the standard Google Play API name for the Open testing track. If our Open test is configured as a custom-named track in the Play Console, this must be set to that exact name instead — please verify before merging.
• Trade-off: Open testing releases go through Google review on each publish (slower than internal). If fast internal iteration is still wanted, we can additionally push to internal in a follow-up.
• The store_metadata lane stays intentionally pinned to internal (it only syncs listing metadata, never a binary) — left untouched.

Context

Pairs with the Play Console fix where app availability was switched back from "Veröffentlichung aufgehoben" to Veröffentlicht (done manually in the Console — that part has no API). This PR ensures that, once the app is available again, the Open test keeps getting the current build automatically.