feat(meditate): Meditate's server, its sealed tables, and SimpleLogin's way back to it

halkyon_meditate_server and its generated client, copied from Todo's
and stripped to the module, the register and the sync engine (ADR 0038,
D1, D7): content_key, meditate_settings and sitting, synced and sealed,
each indexing spaceId and deleted with the person's space; two
migrations, the initial one and sittings (purely additive, the ADR
explains the SQL); ports 8280 to 8282 and PostgreSQL 8290 (D13).
Tests: the module's basics naming `meditate`, the sync engine refusing
calls without a session, the register's modes, and a person's deletion
taking their rows alone.

halkyon_base's forwarding page knows Meditate's ways back
(meditate-loopback on 43826, meditate-scheme app.halkyon.meditate),
which halkyon_app's repository names among the apps' ports.

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

apps/meditate/halkyon_meditate_client/doc/endpoint.md

@@ -0,0 +1,15 @@
+# Callable endpoints
+
+Each class contains callable methods that will call a method on the server side. These are normally defined in the `endpoint` directory in your server project. This client sends requests to these endpoints and returns the result.
+
+Example usage:
+
+```dart
+// How to use GreetingEndpoint.
+client.greeting.hello("world!");
+
+// Generic format.
+client.<endpoint>.<method>(...);
+```
+
+Please see the full official documentation [here](https://docs.serverpod.dev)