feat(train): interval plans, synced and sealed, with an editor and templates (ADR 0030, D42)

Train's server gains the synced `plan` table, a sealed payload alone;
the migration `plans` only adds it, its space index and the foreign key
to its sync space, and a person's deletion takes it with their space.

The app keeps plans as `plan` rows sealed for their row: a name, what
they are for, whether a step off target is told, and blocks of steps
repeated, each ending after a time, a distance or a lap tap and aiming
at a pace, speed, heart rate or zone, or the rower's split, watts or
stroke rate. Home's Plans tab lists them, a new one starts from nothing
or from a template (4×4 minutes, Tabata, 10×400 m), the editor changes
steps on a sheet, and a deletion asks first and can be undone.

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/migrations/migration_registry.txt

@@ -8,3 +8,4 @@
 20261028101012641-workouts
 20261028164640000-profile
 20261028181405893-days
+20261028211508606-plans