feat: add parse endpoint, refactor recurring tasks, and improve web task completion

Extract CreateRecurringTask into engine package for reuse by both CLI
and API. Add POST /tasks/parse endpoint for CLI-style input parsing.
Remove FK constraint on change_log to preserve history after task
deletion. Update web frontend to filter completed tasks from view and
add mock mode support for development.

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

opal-task/internal/engine/recurrence.go

@@ -100,6 +100,94 @@ func CalculateNextDue(currentDue time.Time, recurrence time.Duration) time.Time
 	return currentDue.Add(recurrence)
 }
 
+// CreateRecurringTask creates a recurring task template and its first instance.
+// It validates the recurrence pattern and due date, creates the template with
+// StatusRecurring, then creates the first pending instance linked to it.
+// Returns the first instance task.
+func CreateRecurringTask(description string, mod *Modifier) (*Task, error) {
+	recurPattern := mod.SetAttributes["recur"]
+	if recurPattern == nil {
+		return nil, fmt.Errorf("no recurrence pattern specified")
+	}
+
+	if mod.SetAttributes["due"] == nil {
+		return nil, fmt.Errorf("recurring tasks require a due date (use due:YYYY-MM-DD or due:monday)")
+	}
+
+	duration, err := ParseRecurrencePattern(*recurPattern)
+	if err != nil {
+		return nil, fmt.Errorf("invalid recurrence pattern: %w", err)
+	}
+
+	now := time.Now()
+	template := &Task{
+		UUID:               uuid.New(),
+		Status:             StatusRecurring,
+		Description:        description,
+		Priority:           PriorityDefault,
+		Created:            now,
+		Modified:           now,
+		RecurrenceDuration: &duration,
+		Tags:               []string{},
+	}
+
+	// Build modifier without the recur attribute
+	tempMod := &Modifier{
+		SetAttributes:  make(map[string]*string),
+		AttributeOrder: []string{},
+		AddTags:        mod.AddTags,
+		RemoveTags:     mod.RemoveTags,
+	}
+	for _, key := range mod.AttributeOrder {
+		if key != "recur" {
+			tempMod.SetAttributes[key] = mod.SetAttributes[key]
+			tempMod.AttributeOrder = append(tempMod.AttributeOrder, key)
+		}
+	}
+
+	if err := tempMod.ApplyToNew(template); err != nil {
+		return nil, fmt.Errorf("failed to apply modifiers to template: %w", err)
+	}
+
+	if err := template.Save(); err != nil {
+		return nil, fmt.Errorf("failed to save template: %w", err)
+	}
+
+	for _, tag := range mod.AddTags {
+		if err := template.AddTag(tag); err != nil {
+			return nil, fmt.Errorf("failed to add tag to template: %w", err)
+		}
+	}
+
+	// Create first instance
+	instance := &Task{
+		UUID:        uuid.New(),
+		Status:      StatusPending,
+		Description: description,
+		Priority:    template.Priority,
+		Created:     now,
+		Modified:    now,
+		ParentUUID:  &template.UUID,
+		Due:         template.Due,
+		Wait:        template.Wait,
+		Scheduled:   template.Scheduled,
+		Project:     template.Project,
+		Tags:        []string{},
+	}
+
+	if err := instance.Save(); err != nil {
+		return nil, fmt.Errorf("failed to save first instance: %w", err)
+	}
+
+	for _, tag := range template.Tags {
+		if err := instance.AddTag(tag); err != nil {
+			return nil, fmt.Errorf("failed to add tag to instance: %w", err)
+		}
+	}
+
+	return instance, nil
+}
+
 // SpawnNextInstance creates a new task instance from completed recurring task
 func SpawnNextInstance(completedInstance *Task) error {
 	if completedInstance.ParentUUID == nil {