feat(tasks): show Billable Hours in Task Time Tracking

Add Task.total_billable_hours aggregating billable TimeEntry.duration_seconds (rounded to 2 decimals)
Expose total_billable_hours via Task.to_dict
Update tasks/view.html to display “Billable Hours” when > 0
No schema change; uses existing billable flag and durations

app/models/task.py

@@ -81,6 +81,22 @@ class Task(db.Model):
             return round(total_seconds / 3600, 2)
         except Exception:
             return 0.0
+
+    @property
+    def total_billable_hours(self):
+        """Calculate total billable hours spent on this task"""
+        try:
+            from .time_entry import TimeEntry
+            total_seconds = db.session.query(
+                db.func.sum(TimeEntry.duration_seconds)
+            ).filter(
+                TimeEntry.task_id == self.id,
+                TimeEntry.end_time.isnot(None),
+                TimeEntry.billable == True
+            ).scalar() or 0
+            return round(total_seconds / 3600, 2)
+        except Exception:
+            return 0.0
     
     @property
     def progress_percentage(self):
@@ -220,6 +236,7 @@ class Task(db.Model):
             'started_at': self.started_at.isoformat() if self.started_at else None,
             'completed_at': self.completed_at.isoformat() if self.completed_at else None,
             'total_hours': self.total_hours,
+            'total_billable_hours': self.total_billable_hours,
             'progress_percentage': self.progress_percentage,
             'is_active': self.is_active,
             'is_overdue': self.is_overdue

app/templates/tasks/view.html

@@ -141,6 +141,15 @@
                             </div>
                         </div>
                         {% endif %}
+
+                        {% if task.total_billable_hours > 0 %}
+                        <div class="col-6 col-md-3">
+                            <div class="text-center">
+                                <div class="h4 text-success mb-1">{{ task.total_billable_hours }}</div>
+                                <small class="text-muted">{{ _('Billable Hours') }}</small>
+                            </div>
+                        </div>
+                        {% endif %}
                         
                         {% if task.estimated_hours and task.total_hours > 0 %}
                         <div class="col-6 col-md-3">