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_server/config/generator.yaml

@@ -3,7 +3,14 @@ type: server
 client_package_path: ../halkyon_train_client
 server_test_tools_path: test/integration/test_tools
 
+# Private data sync through serverpod_offline_sync (ADR 0001, ADR 0030's D12): 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