feat(schedule): 任务支持「每天定时 / 每周定时」，用选择器而不是手写 cron

- schedule.py: 新增调度计算模块（纯函数，便于单测）。三种模式：interval（每 N 分钟）/ daily（选钟点）/ weekly（选星期 + 钟点）
- 钟点模式是「固定时刻」而非「固定延迟」——从日历重算，所以某轮跑晚了不会把之后每一轮都拖晚。interval 保持原语义：从上一轮开始计时
- 抖动只加给 interval。给「每天 9:00」也加抖动就成了 9:00–9:01 随机触发，操作者选的时间被悄悄改掉，只会像 bug
- 模型 / schema / service: 新增 schedule_mode / schedule_hours / schedule_days / schedule_minute。时钟字段存逗号分隔文本——几个小整数、永远整体读写，单开一张表只会换来 join。interval_minutes 保留且仍是默认值，已有任务不受影响
- service: 改动任何调度字段都按合并后的状态重算 next_run_at。重新启用也算改动，否则停用一个月再打开会带着一个月前的 next_run_at，一保存就立即触发
- 前端: 运行方式三选一 + 小时/星期胶囊多选 + 分钟下拉，并实时预览结果句子。任务卡片改显示后端拼好的 schedule_label，避免列表和编辑器对同一计划给出两种说法
- 校验: 钟点模式至少选一个时间，按周至少选一个星期

tests/test_schedule.py 新增 24 个用例，含「恰好等于当前时刻的档位归属下一天」这个会让调度器自循环的边界。

api/monitor/models.py

@@ -75,7 +75,7 @@ MODE_NOTE = "note"
 
 
 class MonitorTask(MonitorBase):
-    """One monitored schedule: a set of targets plus an interval."""
+    """One monitored schedule: a set of targets, plus when to run them."""
 
     __tablename__ = "monitor_task"
 
@@ -86,6 +86,21 @@ class MonitorTask(MonitorBase):
     enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
     interval_minutes: Mapped[int] = mapped_column(Integer, nullable=False, default=360)
 
+    # How the task is scheduled. `interval` is the original "every N minutes" and
+    # stays the default; `daily` and `weekly` fire at chosen clock times instead
+    # (the arithmetic lives in schedule.py).
+    #
+    # The clock fields are comma-separated text rather than a child table: they
+    # are a handful of small integers, always read as a whole, and a table would
+    # buy nothing but joins.
+    schedule_mode: Mapped[str] = mapped_column(String(16), nullable=False, default="interval")
+    # 0-23, e.g. "9,12,18". Empty in interval mode.
+    schedule_hours: Mapped[str] = mapped_column(String(96), nullable=False, default="")
+    # 0-6 with Monday = 0, matching Python's date.weekday(). Weekly mode only.
+    schedule_days: Mapped[str] = mapped_column(String(32), nullable=False, default="")
+    # Minute past the hour, shared by every time in the schedule.
+    schedule_minute: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+
     # Crawl window knobs, mirrored onto each run's CLI flags.
     max_notes_count: Mapped[int] = mapped_column(Integer, nullable=False, default=20)
     enable_comments: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)