build(todo): Todo deployable beside Learn's server, and released as an APK, on F-Droid and as a snap (ADR 0033, step 5)

deploy/compose.todo.yaml adds Todo's server at todo.halkyon.eu, a member
of Learn's register, its database halkyon_todo in the instance's
PostgreSQL, its image built as Train's. redeploy.sh and backup.sh go over
every app server .env switches on (train, todo). Each compose file names
its server to the register in a variable of its own
(INSTANCE_MEMBER_TRAIN, INSTANCE_MEMBER_TODO), which halkyon_base adds to
INSTANCE_MEMBERS, each address once, as a second compose file's default
would have replaced Train's.

apps/todo/release.dart configures the release tool for Todo's APK and
snap: a key of its own (pinned once made; until then nothing is
released), the F-Droid listing (Time, NonFreeNet), and the snap from
apps/todo/snap/, which the tool builds in a clone of the repository at the
tag with that folder in place of snap/, as snapcraft builds one recipe a
folder and the root's is Learn's. What's new for 0.1.0 in English and
German, now shown. Android's backup and transfer are off, as Train's
(D53): the reminders scheduled keep their tasks' titles unsealed.

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

packages/halkyon_base_server/README.md

@@ -191,7 +191,9 @@ in itself.
 - **Modes** (`AccountsMode`, `HalkyonBase.accounts`; a host reads it with
   `AccountsMode.fromEnvironment()`): `HoldsRegister` by default, also for an instance of
   one service, telling the members `INSTANCE_MEMBERS` names (web servers, separated by
-  commas or spaces) when an account goes; `MemberOfRegister` where
+  commas or spaces) and each `INSTANCE_MEMBER_<NAME>` names (one each, so that every
+  service's compose file names its own, where one variable would be replaced by the
+  last; each address once) when an account goes; `MemberOfRegister` where
   `ACCOUNT_REGISTER_URL` names the register's web server. Empty values count as unset.
 - **Trust:** the instance's shared secret (`instanceSecret` in the passwords), sent as a
   bearer token with every call between the services and compared in constant time. A