Remove school day hours from schedule templates

The day progress bar no longer uses day start/end times (it uses block
durations instead), so the field is no longer needed.

Removed from: Admin UI, schedule store, schedule model/schemas/router,
session broadcast payload, dashboard snapshot, and startup migrations.
DB columns are left in place (harmless, no migration required).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/routers/schedules.py

@@ -44,8 +44,6 @@ async def create_template(
         name=body.name,
         child_id=body.child_id,
         is_default=body.is_default,
-        day_start_time=body.day_start_time,
-        day_end_time=body.day_end_time,
     )
     db.add(template)
     await db.flush()  # get template.id before adding blocks