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/models/schedule.py

@@ -39,6 +39,7 @@ class ScheduleBlock(Base):
     )
     time_start: Mapped[time] = mapped_column(Time, nullable=False)
     time_end: Mapped[time] = mapped_column(Time, nullable=False)
+    duration_minutes: Mapped[int | None] = mapped_column(Integer, nullable=True)
     label: Mapped[str | None] = mapped_column(String(100), nullable=True)  # override subject name
     notes: Mapped[str | None] = mapped_column(Text, nullable=True)
     order_index: Mapped[int] = mapped_column(Integer, default=0)