feat(todo): the sync engine and Todo's first models (ADR 0033, D5, D6)

Todo's server syncs with serverpod_offline_sync as Learn's and Train's
do, and holds two synced tables, every payload sealed on the devices:
content_key (the person's data key, sealed to the account) and project
(the Inbox or one of the person's own, changed when edited). A person's
space goes with them, with everything in it.

The migration only adds: the two tables, each indexed on spaceId and
cascading with its space (ON DELETE CASCADE to offline_sync_spaces), the
sync engine's own tables (crdt_*, offline_sync_*) with their foreign
keys, the gen_random_uuid_v7() function their ids default to, and the
modules' version rows. Nothing is dropped or changed.

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/todo/halkyon_todo_server/config/generator.yaml

@@ -3,7 +3,14 @@ type: server
 client_package_path: ../halkyon_todo_client
 server_test_tools_path: test/integration/test_tools
 
+# Private data sync through serverpod_offline_sync (ADR 0001, ADR 0033's D5): its
+# module, and the experimental `database: sync` for the models it syncs.
 modules:
+  serverpod_offline_sync:
+    nickname: offline_sync
   # What every Halkyon server has (ADR 0014): `client.modules.base` in the app.
   halkyon_base:
     nickname: base
+
+experimental_features:
+  databaseSync: true