Add time-based day progress bar to dashboards

Replaces block-count progress with a wall-clock progress bar driven by
configurable day start/end hours on each schedule template.

- ScheduleTemplate: add day_start_time / day_end_time (TIME, nullable)
- Startup migration: idempotent ALTER TABLE for existing DBs
- Dashboard snapshot: includes day_start_time / day_end_time from template
- Admin → Schedules: time pickers in block editor to set day hours
- Dashboard view: time-based progress bar with start/current/end labels
- TV view: full-width day progress strip between header and main content

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

backend/app/schemas/session.py

@@ -1,4 +1,4 @@
-from datetime import date, datetime
+from datetime import date, datetime, time
 from pydantic import BaseModel
 from app.schemas.schedule import ScheduleBlockOut
 from app.schemas.child import ChildOut
@@ -43,3 +43,5 @@ class DashboardSnapshot(BaseModel):
     blocks: list[ScheduleBlockOut] = []
     completed_block_ids: list[int] = []
     block_elapsed_seconds: int = 0  # seconds already elapsed for the current block
+    day_start_time: time | None = None
+    day_end_time: time | None = None