tasks: add refresh mode toggle (completion date vs planned date)

Recurring tasks can now calculate next due date from either the
completion time (default) or the planned due date, catching up
if overdue.

src/models/Task.ts

@@ -7,6 +7,7 @@ export interface ITask {
   assignees: string[];
   tags: string[];
   difficulty?: 'low' | 'medium' | 'high';
+  refreshMode?: 'completion' | 'planned';
   isRecurring: boolean;
   frequency?: {
     type: 'daily' | 'weekly' | 'biweekly' | 'monthly' | 'custom';
@@ -45,6 +46,11 @@ const TaskSchema = new mongoose.Schema(
       type: String,
       enum: ['low', 'medium', 'high']
     },
+    refreshMode: {
+      type: String,
+      enum: ['completion', 'planned'],
+      default: 'completion'
+    },
     isRecurring: {
       type: Boolean,
       required: true,