feat(train): the Loop's days read on the phone (ADR 0030, D37, D38)

The server gains Train's synced `day`, `day_series` (a day's heart
rate, deleted with its day) and `night`, each a sealed payload alone;
the migration `days` only adds their tables, indexes and foreign keys
(to their sync space, and a chunk's to its day, deferred), and the
person's deletion takes them with their spaces.

On Android, `LoopDays.kt` reads a band's days from a date to today,
framed by the SDK's sync start and stop, into lines of JSON, then
deletes on the band what the app kept up to the date it names, never
today; `LoopBridge` answers `readDays` with them, one band call at a
time, and `background` schedules `LoopWorker`, a WorkManager job every
four hours that appends the same lines to the person's
`loop_days.txt` while the app doesn't hold the band.

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/lib/migrations/migration_registry.dart

@@ -6,6 +6,7 @@ import 'package:serverpod_database/serverpod_database.dart';
 
 part '20261028101012641-workouts/migration.dart';
 part '20261028164640000-profile/migration.dart';
+part '20261028181405893-days/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -14,6 +15,7 @@ class MigrationRegistry {
   static final List<MigrationVersionSql> migrations = [
     _Migration20261028101012641_workouts(),
     _Migration20261028164640000_profile(),
+    _Migration20261028181405893_days(),
   ];
 
   /// List of all client-side database migration versions.