Add duration_minutes field to schedule blocks

Separates "recommended time" (time_start/time_end) from actual timer
duration. If duration_minutes is set, the timer counts down from that
value; otherwise falls back to the time_start–time_end window.

- ScheduleBlock model: add nullable duration_minutes INT column
- Startup migration: idempotent ADD COLUMN for existing DBs
- Schemas: duration_minutes in create, update, and out
- TimerDisplay: prefer duration_minutes * 60 over time diff when set
- Admin block forms: Duration (min) input on add and edit forms

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

backend/app/main.py

@@ -31,6 +31,7 @@ async def lifespan(app: FastAPI):
         # Idempotent column additions for schema migrations
         await _add_column_if_missing(conn, "schedule_templates", "day_start_time", "TIME NULL")
         await _add_column_if_missing(conn, "schedule_templates", "day_end_time", "TIME NULL")
+        await _add_column_if_missing(conn, "schedule_blocks", "duration_minutes", "INT NULL")
     yield