feat(train): the sync engine and Train's first models (ADR 0030, D14)

Train's server syncs with serverpod_offline_sync as Learn's does, and
holds three synced tables, every payload sealed on the devices:
content_key (the person's data key, sealed to the account), workout (its
type, times, name and totals, changed when edited) and workout_series
(chunks of a workout's samples, written once). A person's space goes
with them, with everything in it.

The migration only adds: the three tables, each indexed on spaceId and
cascading with its space (a series also with its workout), and the sync
engine's own tables.

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/train/halkyon_train_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