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/schemas/session.py

@@ -1,4 +1,4 @@
-from datetime import date, datetime, time
+from datetime import date, datetime
 from pydantic import BaseModel
 from app.schemas.schedule import ScheduleBlockOut
 from app.schemas.child import ChildOut
@@ -44,7 +44,5 @@ class DashboardSnapshot(BaseModel):
     completed_block_ids: list[int] = []
     block_elapsed_seconds: int = 0  # seconds already elapsed for the current block
     is_paused: bool = False          # whether the current block's timer is paused
-    day_start_time: time | None = None
-    day_end_time: time | None = None
     morning_routine: list[str] = []  # text items shown on TV during greeting state
     break_activities: list[str] = []  # text items shown on TV during break time