fix(db): _ensure_columns 改为按模型元数据推导，并补上漏加的调度字段

上一个提交加了 4 个调度字段，却没在 _ADDED_COLUMNS 里登记，后果是生产环境：
  - 任务列表接口报 Unknown column，UI 打不开任务列表
  - 调度器每 20 秒 tick 一次炸一次，定时任务完全不会触发
  最阴险的是启动完全正常——能连库、能起来，只是随后每条查询都失败。

- _ensure_columns 不再遍历手写清单，改为遍历 MonitorBase.metadata.sorted_tables，
  从根上消掉「加了字段忘了登记」这类漏
- 新增 _column_ddl：用 CreateColumn 渲染类型与可空性，并给 NOT NULL 列补 DEFAULT。
  模型的 default= 是 ORM 侧行为、不会进 DDL，而给已有数据的表加 NOT NULL 列必须有值，
  否则能否成功取决于服务端 sql_mode
- 主键列跳过：MySQL 不允许 AUTO_INCREMENT 与 DEFAULT 共存

新增 tests/test_monitor_column_migration.py 守住：每个 NOT NULL 列都必须能生成带
DEFAULT 的合法 ALTER。

api/monitor/db.py

@@ -44,7 +44,9 @@ from contextlib import asynccontextmanager
 from pathlib import Path
 from typing import AsyncIterator, Optional
 
-from sqlalchemy import event, text
+from sqlalchemy import Column, event, text
+from sqlalchemy.dialects import mysql
+from sqlalchemy.schema import CreateColumn
 from sqlalchemy.ext.asyncio import (
     AsyncEngine,
     AsyncSession,
@@ -214,14 +216,53 @@ async def init_db() -> None:
         await _migrate_setting_keys(conn)
 
 
-# Columns added to a table after it may already exist. ``create_all`` only
-# creates missing *tables*, so new columns need an explicit ALTER TABLE.
-_ADDED_COLUMNS: dict[str, list[tuple[str, str]]] = {
-    "monitor_task": [
-        ("notify_enabled", "BOOLEAN NOT NULL DEFAULT 0"),
-        ("last_notified_at", "BIGINT NULL"),
-    ],
-}
+# ``create_all`` creates missing *tables* but never adds *columns* to a table that
+# already exists, so those need an explicit ALTER TABLE.
+#
+# Which columns those are is derived from the ORM metadata, not kept by hand. The
+# hand-kept version was a trap: forgetting to register a new column there still let
+# the app start -- it connects fine, then fails on every query and every scheduler
+# tick. Which is exactly what happened when the scheduling columns were added.
+
+
+def _implicit_default(column: Column) -> Optional[str]:
+    """A literal to seed existing rows with when a NOT NULL column is added."""
+    default = column.default
+    if default is not None and getattr(default, "is_scalar", False):
+        value = default.arg
+        if isinstance(value, bool):
+            return "1" if value else "0"
+        if isinstance(value, (int, float)):
+            return str(value)
+        return "'" + str(value).replace("'", "''") + "'"
+
+    # No scalar default on the model. Fall back to the type's zero value, so that
+    # adding the column cannot depend on the server's sql_mode.
+    try:
+        python_type = column.type.python_type
+    except NotImplementedError:
+        return None
+    if python_type in (bool, int, float):
+        return "0"
+    if python_type is str:
+        return "''"
+    return None
+
+
+def _column_ddl(column: Column) -> str:
+    """One column as MySQL DDL for ``ALTER TABLE ... ADD COLUMN``.
+
+    ``CreateColumn`` renders the name, type and nullability. The default is added
+    separately because a model's ``default=`` is applied by the ORM and never
+    reaches the DDL -- and a NOT NULL column added to a populated table needs a
+    value for the rows already sitting there.
+    """
+    ddl = str(CreateColumn(column).compile(dialect=mysql.dialect()))
+    if not column.nullable and column.server_default is None:
+        seed = _implicit_default(column)
+        if seed is not None:
+            ddl += f" DEFAULT {seed}"
+    return ddl
 
 
 async def _existing_columns(conn, table: str) -> set[str]:
@@ -240,14 +281,22 @@ async def _existing_columns(conn, table: str) -> set[str]:
 
 
 async def _ensure_columns(conn) -> None:
-    for table, columns in _ADDED_COLUMNS.items():
-        existing = await _existing_columns(conn, table)
+    """Add every model column the live table is missing."""
+    for table in MonitorBase.metadata.sorted_tables:
+        existing = await _existing_columns(conn, table.name)
         if not existing:
             # Table did not exist before this run; create_all built it complete.
             continue
-        for name, ddl in columns:
-            if name not in existing:
-                await conn.execute(text(f"ALTER TABLE {table} ADD COLUMN {name} {ddl}"))
+        for column in table.columns:
+            # Primary keys are always present, and MySQL rejects AUTO_INCREMENT
+            # alongside the DEFAULT this helper appends -- so skip them rather
+            # than emit DDL that could never run.
+            if column.name in existing or column.primary_key:
+                continue
+            print(f"[monitor.db] 补齐缺失字段 {table.name}.{column.name}", flush=True)
+            await conn.execute(
+                text(f"ALTER TABLE {table.name} ADD COLUMN {_column_ddl(column)}")
+            )
 
 
 async def _migrate_setting_keys(conn) -> None: