feat: a release script for the APK and the snap

tool/release.sh builds a release on the maintainer's laptop: one APK for
every phone, from a checkout of the signed tag and signed with the app's
own key, and the snap, both connecting to learn.halkyon.eu, the project's
instance; their checksums signed with GPG; the release on code.halkyon.app
through Forgejo's API, and the snap in the Store's edge channel.

Before building it checks that main is clean and pushed, that the tag is
signed by a key deploys accept, the version and a raised build number,
the pinned Flutter, and that the keystore holds the key whose certificate
tool/release/android-certificate.sha256 names. After building it checks
the APK's signer, package, version, that it isn't debuggable and that it
names the server, and the snap's version. The key's password and the
Forgejo token are asked for at each run and never stored.

Release builds take the app's key from the environment and otherwise fall
back to the debug key, so `flutter run --release` works anywhere. The
native libraries go into the APK compressed: an APK for three
architectures was 110 MB with them stored as they are, 48 MB now. The
list of dependencies Gradle adds for Google Play is left out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

AGENTS.md

@@ -120,6 +120,7 @@ A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS s
 - **CI** (`.forgejo/workflows/ci.yaml`) runs `tool/check.sh` on every push, on the Forgejo runner on sahaquiel (`deploy/runner/`). Jobs run directly on the host as the unprivileged user `runner`, never in containers: Docker access would be root on the production server. The runner's systemd unit keeps jobs away from production (no `/home`, no Docker socket, Docker's container networks unreachable, memory and CPU capped); keep it that way when changing it. `runner` has user ID 61000 and must never share one with a container's processes (PostgreSQL and the server run as 999, not remapped): the same user ID can read their environment and files. The job's first step (*Sandbox*) fails if any of this leaks. Flutter is pinned in the workflow (`FLUTTER_VERSION`): move it together with the local one. The server tests there get throwaway passwords from the environment (`SERVERPOD_…`, new for every job), since `passwords.yaml` is never committed; without them they exit with code 1 and no message (Serverpod's own template does the same). CI also runs `serverpod generate` and fails if the committed generated code differs from its output. CI runs on pushes only: a pull request from a fork would run its code on the production server.
 - **A green `main` deploys learn.halkyon.eu** through `deploy/redeploy.sh`, the only command the runner's key may run as `deploy` (pinned in `authorized_keys`, from sahaquiel itself only). It deploys the commit CI checked while it is still the tip, as a fast-forward, only if every new commit is signed by a key in `deploy/allowed-signers` (the deployed copy decides), after dumping the database (the last 3 dumps stay in `~deploy/backups`: the way back from a deploy, not backups), applies the migrations in a run of their own before switching (`--role=maintenance --apply-migrations`: a failing migration stops the deploy while the old server keeps serving, so migrations must keep the old server working until the switch), and fails unless the new server answers. A failed deploy leaves the checkout at the deployed commit; run the workflow again to retry. By hand: `ssh sahaquiel-deploy halkyon-learn/deploy/redeploy.sh <commit>`. Never `docker compose down -v` there.
 - **Backups** (`deploy/backups.md`): every hour, `deploy/backup.sh` (a systemd timer, as `deploy`) dumps the database, which holds people's files too, and restic stores it in a Hetzner Storage Box (24 hourly, 7 daily, 4 weekly, 3 monthly); the box's own snapshots protect it from a taken-over server. A restore takes the instance back up to an hour, and people who started since lose what their device held: their app no longer finds its session, starts a new anonymous account and deletes the old folder. So a change that makes devices lose their session, or keeps deleted people's data longer, changes what a restore costs; say so in `deploy/backups.md`.
+- **Releases** are the project's, not an instance's: the release, the Snap Store listing and the website point to learn.halkyon.app; the builds connect to learn.halkyon.eu, the project's instance, until the app lets people choose one. They come from a developer's laptop, never from CI: `tool/release.sh vX.Y.Z` on `main` as pushed, with the tag signed by a key in `deploy/allowed-signers` and pushed, and the build number raised. It builds one universal APK from a checkout of the tag, signed with the app's key, and the snap (`snap/snapcraft.yaml`, the committed HEAD, built in LXD), both with `SERVER_URL=https://learn.halkyon.eu/`; signs `SHA256SUMS` with GPG; creates the release on code.halkyon.app through Forgejo's API; and uploads the snap to the Store's edge channel. The key's password and the Forgejo token are asked for at each run and never stored. The app's key is a PKCS #12 keystore outside the repository (also in the maintainer's password manager); there is no Google Play, so it is the app's identity: lost, installed apps can't be updated. Its certificate's SHA-256 is in `tool/release/android-certificate.sha256`, and the script releases no APK signed otherwise; other release builds fall back to the debug key (`android/app/build.gradle.kts`), so `flutter run --release` works anywhere.
 
 ## Rules the code relies on