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/schemas/monitor.py

@@ -20,7 +20,7 @@
 
 from typing import List, Literal, Optional
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, model_validator
 
 # A floor on the interval is a correctness guard, not a nicety: every run
 # launches a browser and hits XHS with several requests, so a short interval
@@ -40,6 +40,19 @@ class MonitorTaskCreate(BaseModel):
     interval_minutes: Optional[int] = Field(
         default=None, ge=MIN_INTERVAL_MINUTES, le=MAX_INTERVAL_MINUTES
     )
+    # --- Scheduling ---------------------------------------------------------
+    # All three modes are expressible with pickers; a raw cron string is
+    # deliberately not supported, since it is a small language to learn just to
+    # say "every day at nine".
+    schedule_mode: Literal["interval", "daily", "weekly"] = "interval"
+    # 0-23, e.g. [9, 12, 18]. Required for the two clock modes.
+    schedule_hours: List[int] = Field(default_factory=list)
+    # 0-6 with Monday = 0, matching Python's date.weekday(). Required for weekly.
+    schedule_days: List[int] = Field(default_factory=list)
+    # One minute for the whole schedule, so a task with three times is
+    # "09:30, 12:30, 18:30" rather than three separate minute choices.
+    schedule_minute: int = Field(default=0, ge=0, le=59)
+
     max_notes_count: Optional[int] = Field(default=None, ge=1, le=500)
     enable_comments: bool = True
     # Raising this widens the comment window, which is the only lever available
@@ -53,6 +66,20 @@ class MonitorTaskCreate(BaseModel):
     # Raw pasted values: full URLs or bare ids, in either form.
     targets: List[str] = Field(min_length=1)
 
+    @model_validator(mode="after")
+    def _validate_schedule(self) -> "MonitorTaskCreate":
+        if any(hour < 0 or hour > 23 for hour in self.schedule_hours):
+            raise ValueError("小时必须在 0-23 之间")
+        if any(day < 0 or day > 6 for day in self.schedule_days):
+            raise ValueError("星期必须在 0-6 之间（周一为 0）")
+        # A clock mode with no chosen time can never fire. Rejecting it here is
+        # what keeps next_occurrence()'s None branch unreachable in practice.
+        if self.schedule_mode in ("daily", "weekly") and not self.schedule_hours:
+            raise ValueError("按钟点调度至少要选一个时间")
+        if self.schedule_mode == "weekly" and not self.schedule_days:
+            raise ValueError("按周调度至少要选一个星期")
+        return self
+
 
 class MonitorTaskUpdate(BaseModel):
     name: Optional[str] = Field(default=None, min_length=1, max_length=200)
@@ -60,6 +87,12 @@ class MonitorTaskUpdate(BaseModel):
     interval_minutes: Optional[int] = Field(
         default=None, ge=MIN_INTERVAL_MINUTES, le=MAX_INTERVAL_MINUTES
     )
+    # None means "leave alone". Cross-field validity depends on the merged state,
+    # so it is checked in the service rather than here.
+    schedule_mode: Optional[Literal["interval", "daily", "weekly"]] = None
+    schedule_hours: Optional[List[int]] = None
+    schedule_days: Optional[List[int]] = None
+    schedule_minute: Optional[int] = Field(default=None, ge=0, le=59)
     max_notes_count: Optional[int] = Field(default=None, ge=1, le=500)
     enable_comments: Optional[bool] = None
     max_comments_count: Optional[int] = Field(default=None, ge=1, le=500)