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/service.py

@@ -28,7 +28,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from tools.time_util import get_current_timestamp
 
-from . import app_settings, platforms
+from . import app_settings, platforms, schedule
 from .db import get_session
 from .platforms import PLATFORM_XHS
 from .models import (
@@ -53,6 +53,28 @@ _background_runs: set[asyncio.Task] = set()
 MIN_INTERVAL_MINUTES = 30
 MAX_INTERVAL_MINUTES = 7 * 24 * 60
 
+# Changing any of these invalidates the pending run slot.
+SCHEDULE_FIELDS = {
+    "interval_minutes",
+    "schedule_mode",
+    "schedule_hours",
+    "schedule_days",
+    "schedule_minute",
+}
+
+
+def _require_clock_fields(mode: str, hours: list, days: list) -> None:
+    """A clock schedule with no clock time can never fire.
+
+    The create schema already rejects that shape, but the update path merges
+    partial fields and therefore has no schema-level view of the result -- so the
+    check lives here, where both paths meet.
+    """
+    if mode in schedule.CLOCK_MODES and not hours:
+        raise ValueError("按钟点调度至少要选一个时间")
+    if mode == schedule.MODE_WEEKLY and not days:
+        raise ValueError("按周调度至少要选一个星期")
+
 _CREATOR_URL_RE = re.compile(r"xiaohongshu\.com/user/profile/([A-Za-z0-9_-]+)")
 _NOTE_URL_RE = re.compile(r"xiaohongshu\.com/(?:explore|discovery/item)/([A-Za-z0-9_-]+)")
 # XHS user ids and note ids are 24-char hex; allow a slightly wider range so a
@@ -139,7 +161,12 @@ async def create_task(session: AsyncSession, payload: Dict[str, Any]) -> Monitor
     # the Settings page actually governs new tasks.
     defaults = await app_settings.defaults(session, platform)
     interval_minutes = payload.get("interval_minutes") or defaults["interval_minutes"]
-    interval_ms = int(interval_minutes) * 60_000
+
+    schedule_mode = payload.get("schedule_mode") or schedule.MODE_INTERVAL
+    schedule_hours = list(payload.get("schedule_hours") or [])
+    schedule_days = list(payload.get("schedule_days") or [])
+    schedule_minute = int(payload.get("schedule_minute") or 0)
+    _require_clock_fields(schedule_mode, schedule_hours, schedule_days)
 
     task = MonitorTask(
         name=payload["name"],
@@ -147,12 +174,23 @@ async def create_task(session: AsyncSession, payload: Dict[str, Any]) -> Monitor
         mode=mode,
         enabled=payload.get("enabled", True),
         interval_minutes=interval_minutes,
+        schedule_mode=schedule_mode,
+        schedule_hours=schedule.format_hours(schedule_hours),
+        schedule_days=schedule.format_days(schedule_days),
+        schedule_minute=schedule_minute,
         max_notes_count=payload.get("max_notes_count") or defaults["max_notes_count"],
         enable_comments=payload.get("enable_comments", True),
         max_comments_count=payload.get("max_comments_count") or defaults["max_comments_count"],
         run_timeout_seconds=payload.get("run_timeout_seconds", 3600),
         notify_enabled=payload.get("notify_enabled", False),
-        next_run_at=now + interval_ms,
+        next_run_at=schedule.next_occurrence(
+            mode=schedule_mode,
+            interval_minutes=interval_minutes,
+            hours=schedule_hours,
+            days=schedule_days,
+            minute=schedule_minute,
+            after_ms=now,
+        ),
         last_status="idle",
         created_at=now,
         updated_at=now,
@@ -189,10 +227,14 @@ async def update_task(session: AsyncSession, task_id: int, payload: Dict[str, An
     if task is None:
         raise ValueError(f"Task {task_id} not found")
 
+    was_enabled = task.enabled
+
     for field in (
         "name",
         "enabled",
         "interval_minutes",
+        "schedule_mode",
+        "schedule_minute",
         "max_notes_count",
         "enable_comments",
         "max_comments_count",
@@ -202,6 +244,13 @@ async def update_task(session: AsyncSession, task_id: int, payload: Dict[str, An
         if field in payload and payload[field] is not None:
             setattr(task, field, payload[field])
 
+    # The clock lists are stored as comma-separated text, so they cannot go
+    # through the generic loop above.
+    if payload.get("schedule_hours") is not None:
+        task.schedule_hours = schedule.format_hours(payload["schedule_hours"])
+    if payload.get("schedule_days") is not None:
+        task.schedule_days = schedule.format_days(payload["schedule_days"])
+
     # Replacing targets resets the baseline implicitly: a note set that now
     # includes new ids will simply report them as new on the next run.
     if payload.get("targets") is not None:
@@ -227,8 +276,23 @@ async def update_task(session: AsyncSession, task_id: int, payload: Dict[str, An
                 )
             )
 
-    if "interval_minutes" in payload and payload["interval_minutes"]:
-        task.next_run_at = get_current_timestamp() + payload["interval_minutes"] * 60_000
+    # Any change to when the task runs invalidates the pending slot, so recompute
+    # it from the merged state rather than working out which field moved.
+    # Re-enabling counts as a change too: otherwise a task switched off for a
+    # month comes back holding a next_run_at a month in the past and fires the
+    # instant it is saved.
+    if (SCHEDULE_FIELDS & set(payload)) or (task.enabled and not was_enabled):
+        hours = schedule.parse_hours(task.schedule_hours)
+        days = schedule.parse_days(task.schedule_days)
+        _require_clock_fields(task.schedule_mode, hours, days)
+        task.next_run_at = schedule.next_occurrence(
+            mode=task.schedule_mode,
+            interval_minutes=task.interval_minutes,
+            hours=hours,
+            days=days,
+            minute=task.schedule_minute,
+            after_ms=get_current_timestamp(),
+        )
 
     task.updated_at = get_current_timestamp()
     await session.flush()
@@ -623,6 +687,7 @@ async def list_tasks(
             "mode": task.mode,
             "enabled": task.enabled,
             "interval_minutes": task.interval_minutes,
+            **_schedule_fields(task),
             "max_notes_count": task.max_notes_count,
             "enable_comments": task.enable_comments,
             "max_comments_count": task.max_comments_count,
@@ -644,6 +709,29 @@ async def list_tasks(
     ]
 
 
+def _schedule_fields(task: MonitorTask) -> Dict[str, Any]:
+    """The schedule columns, plus the sentence the task list renders.
+
+    The label is composed here rather than in the frontend so the list and the
+    editor cannot drift on what a given schedule means.
+    """
+    hours = schedule.parse_hours(task.schedule_hours)
+    days = schedule.parse_days(task.schedule_days)
+    return {
+        "schedule_mode": task.schedule_mode,
+        "schedule_hours": hours,
+        "schedule_days": days,
+        "schedule_minute": task.schedule_minute,
+        "schedule_label": schedule.describe(
+            mode=task.schedule_mode,
+            interval_minutes=task.interval_minutes,
+            hours=hours,
+            days=days,
+            minute=task.schedule_minute,
+        ),
+    }
+
+
 async def overview(session: AsyncSession, platform: Optional[str] = None) -> Dict[str, Any]:
     """Headline numbers for the dashboard tiles, scoped to one platform."""
     now = get_current_timestamp()