feat(accounts): one account across an instance's services (ADR 0016)

halkyon_base runs as the instance's register or as one of its members.
The register keeps the ways in, handles and profiles; members ask it
over HTTP with the instance's shared secret (instanceSecret), and open
sessions of their own for its accounts, under the same account ID.

- AccountsMode from the environment: INSTANCE_MEMBERS on the register,
  ACCOUNT_REGISTER_URL on a member. Learn always holds the register
  (Explore joins the profiles); Todo is a member where it is set.
- AccountRegister: LocalRegister, RemoteRegister, an in-memory one in
  tests. Calls between services on the web server, refused without the
  secret (constant-time compare); a server needing it doesn't start
  without it.
- Deleting an account deletes it everywhere: notices per member kept in
  the deletion's transaction (account_deletion_notice), told at once and
  hourly until confirmed; members prune daily whom the register no longer
  knows, never while it knows none of them or can't be reached.
- Deploy: INSTANCE_SECRET and INSTANCE_MEMBERS in compose and .env.

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

deploy/Caddyfile

@@ -1,6 +1,7 @@
 # Caddy forwards to the API server, and the assistant connector's paths (ADR 0006),
 # profile pages (ADR 0011) and the dictionaries devices download (ADR 0012) to the web
-# server (8082), which serves nothing else. Insights (8081) doesn't run in production;
+# server (8082), which serves nothing else. Never the calls between the instance's
+# services (`/halkyon-base/…`, ADR 0016): they reach each other on its own network. Insights (8081) doesn't run in production;
 # if it is ever re-enabled, reach it through an SSH tunnel, never through this file.
 #
 # {$DOMAIN} is `learn.halkyon.eu` on its server, where Caddy obtains and renews a