chore(android): add persistent release-signing keystore generator

An Android APK's signature identifies the app to the OS; a new build must
carry the same one as the last install or Android refuses it as an update.
generate-keystore.sh creates the keystore once, interactively, and refuses
to run again once one exists to avoid ever changing it by accident. The
resulting file is git-ignored — it's a secret that must live outside the
repo, backed up separately.

docker/keystore/README.md

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+# Android release-signing keystore
+
+This folder holds the persistent keystore used to sign release APKs built by
+`docker/build.sh android`. Every release must be signed with the **same**
+key, or Android refuses to install a new build over a previously installed
+one (`INSTALL_FAILED_UPDATE_INCOMPATIBLE`) and forces the user to uninstall
+first — which deletes the app's data.
+
+## First-time setup
+
+```bash
+./docker/keystore/generate-keystore.sh
+```
+
+Run this **once**, on whichever machine will keep producing release builds.
+It creates `release.keystore` in this folder and asks for a keystore
+password (and, optionally, a separate key password). Nothing is written to
+git — `release.keystore` is ignored by `.gitignore`, and the script never
+saves the passwords anywhere.
+
+`docker/build.sh android` will refuse to run without this file, and will
+prompt you for the same passwords at every build.
+
+## Back it up
+
+The keystore file and its passwords cannot be regenerated: if lost, no
+future release can ever update existing installs in place again — every
+user would need to uninstall and reinstall from scratch. Copy
+`release.keystore` and the passwords to a password manager or encrypted
+backup outside this repository as soon as you generate them.
+
+## What NOT to do
+
+- Do not commit `release.keystore` (or any `*.jks`/`*.keystore` file) to git.
+- Do not re-run `generate-keystore.sh` once a keystore already exists — it
+  refuses on purpose; delete the file yourself only if you fully accept the
+  consequences above.