feat: 添加工作任务管理功能

- 新增 Task 模型（状态、优先级、进展百分比）
- 任务 API（列表、统计、进展更新、完成标记）
- 前端任务板块（统计卡片 + 任务列表）
- 进展可视化（进度条 + 进展记录）

backend/diary/models.py

@@ -64,3 +64,59 @@ class DailyProgress(models.Model):
 
     def __str__(self):
         return f"{self.entry.date} - {self.category}: {self.progress_percent}%"
+
+
+class Task(models.Model):
+    """工作任务 - 跟踪任务和进展"""
+    STATUS_CHOICES = [
+        ('pending', '⏳ 待开始'),
+        ('in_progress', '🔄 进行中'),
+        ('blocked', '🚧 已阻塞'),
+        ('completed', '✅ 已完成'),
+        ('cancelled', '❌ 已取消'),
+    ]
+
+    PRIORITY_CHOICES = [
+        ('low', '低'),
+        ('medium', '中'),
+        ('high', '高'),
+        ('critical', '紧急'),
+    ]
+
+    title = models.CharField('任务标题', max_length=200)
+    description = models.TextField('任务描述', blank=True, default='')
+    status = models.CharField('状态', max_length=20, choices=STATUS_CHOICES, default='pending')
+    priority = models.CharField('优先级', max_length=20, choices=PRIORITY_CHOICES, default='medium')
+    progress_percent = models.IntegerField('进展百分比', default=0)
+    progress_notes = models.TextField('进展记录', blank=True, default='')
+    assigned_to = models.CharField('负责人', max_length=100, blank=True, default='码神')
+    due_date = models.DateField('截止日期', null=True, blank=True)
+    completed_at = models.DateTimeField('完成时间', null=True, blank=True)
+    created_at = models.DateTimeField('创建时间', auto_now_add=True)
+    updated_at = models.DateTimeField('更新时间', auto_now=True)
+
+    class Meta:
+        ordering = ['-priority', 'created_at']
+        verbose_name = '任务'
+        verbose_name_plural = '任务'
+
+    def __str__(self):
+        return f"{self.title} ({self.get_status_display()})"
+
+    def mark_completed(self):
+        """标记任务为已完成"""
+        self.status = 'completed'
+        self.progress_percent = 100
+        self.completed_at = timezone.now()
+        self.save()
+
+    def update_progress(self, percent, notes=''):
+        """更新任务进展"""
+        self.progress_percent = percent
+        if percent > 0 and self.status == 'pending':
+            self.status = 'in_progress'
+        if percent == 100:
+            self.mark_completed()
+        if notes:
+            self.progress_notes = f"[{timezone.now().strftime('%Y-%m-%d %H:%M')}] {notes}\n" + self.progress_notes
+        self.save()