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/schemas/schedule.py

@@ -6,6 +6,7 @@ class ScheduleBlockCreate(BaseModel):
     subject_id: int | None = None
     time_start: time
     time_end: time
+    duration_minutes: int | None = None
     label: str | None = None
     notes: str | None = None
     order_index: int = 0
@@ -15,6 +16,7 @@ class ScheduleBlockUpdate(BaseModel):
     subject_id: int | None = None
     time_start: time | None = None
     time_end: time | None = None
+    duration_minutes: int | None = None
     label: str | None = None
     notes: str | None = None
     order_index: int | None = None
@@ -25,6 +27,7 @@ class ScheduleBlockOut(BaseModel):
     subject_id: int | None
     time_start: time
     time_end: time
+    duration_minutes: int | None
     label: str | None
     notes: str | None
     order_index: int