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_client/pubspec.yaml

@@ -13,3 +13,4 @@ dependencies:
     path: ../../../packages/halkyon_base_client
   serverpod_auth_idp_client: 4.0.4
   serverpod_client: 4.0.4
+  serverpod_offline_sync_client: 0.0.10