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>

frontend/src/components/TimerDisplay.vue

@@ -45,6 +45,7 @@ function parseTime(str) {
 
 const blockDuration = computed(() => {
   if (!props.block) return 0
+  if (props.block.duration_minutes != null) return props.block.duration_minutes * 60
   return parseTime(props.block.time_end) - parseTime(props.block.time_start)
 })
 

frontend/src/views/AdminView.vue

@@ -168,6 +168,12 @@
                     <input v-model="editingBlock.time_start" type="time" required />
                     <span>to</span>
                     <input v-model="editingBlock.time_end" type="time" required />
+                    <input
+                      v-model.number="editingBlock.duration_minutes"
+                      type="number" min="1" max="480"
+                      placeholder="Duration (min)"
+                      style="width:130px"
+                    />
                     <input v-model="editingBlock.label" placeholder="Label (optional)" />
                     <button type="submit" class="btn-sm btn-primary">Save</button>
                     <button type="button" class="btn-sm" @click="editingBlock = null">Cancel</button>
@@ -192,6 +198,12 @@
                 <input v-model="newBlock.time_start" type="time" required />
                 <span>to</span>
                 <input v-model="newBlock.time_end" type="time" required />
+                <input
+                  v-model.number="newBlock.duration_minutes"
+                  type="number" min="1" max="480"
+                  placeholder="Duration (min)"
+                  style="width:130px"
+                />
                 <input v-model="newBlock.label" placeholder="Label (optional)" />
                 <button type="submit" class="btn-primary btn-sm">Add Block</button>
               </form>
@@ -295,7 +307,7 @@ const templates = ref([])
 const showCreateForm = ref(false)
 const editingTemplate = ref(null)
 const newTemplate = ref({ name: '', child_id: null, is_default: false })
-const newBlock = ref({ subject_id: null, time_start: '', time_end: '', label: '', order_index: 0 })
+const newBlock = ref({ subject_id: null, time_start: '', time_end: '', duration_minutes: null, label: '', order_index: 0 })
 const editingBlock = ref(null)
 
 function childName(id) {
@@ -331,7 +343,7 @@ async function addBlock(templateId) {
     order_index: templates.value.find((t) => t.id === templateId)?.blocks.length || 0,
   }
   await api.post(`/api/schedules/${templateId}/blocks`, payload)
-  newBlock.value = { subject_id: null, time_start: '', time_end: '', label: '', order_index: 0 }
+  newBlock.value = { subject_id: null, time_start: '', time_end: '', duration_minutes: null, label: '', order_index: 0 }
   await loadTemplates()
 }
 
@@ -341,6 +353,7 @@ function startEditBlock(block) {
     subject_id: block.subject_id,
     time_start: block.time_start ? block.time_start.slice(0, 5) : '',
     time_end: block.time_end ? block.time_end.slice(0, 5) : '',
+    duration_minutes: block.duration_minutes ?? null,
     label: block.label || '',
   }
 }