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:

tests/test_monitor_column_migration.py

@@ -0,0 +1,78 @@
+# -*- coding: utf-8 -*-
+"""Guards for the in-place column migration.
+
+``create_all`` creates missing tables but never adds columns to a table that
+already exists, so new model columns are applied by ``_ensure_columns``. That step
+was originally driven by a hand-kept list, and forgetting to update it did not
+fail loudly -- the app still started, connected, and then failed on every query
+and every scheduler tick. These tests pin down its replacement, which derives the
+work from the model metadata.
+"""
+
+import pytest
+from sqlalchemy import Boolean, Column, Integer, MetaData, String, Table
+
+from api.monitor import db
+from api.monitor.models import MonitorBase
+
+
+def _migratable_columns():
+    for table in MonitorBase.metadata.sorted_tables:
+        for column in table.columns:
+            if column.primary_key:
+                continue
+            yield pytest.param(column, id=f"{table.name}.{column.name}")
+
+
+def _ddl(column: Column) -> str:
+    """Render a detached column, so the tests never mutate the real metadata."""
+    scratch = Table("scratch", MetaData(), column)
+    return db._column_ddl(scratch.columns[0])
+
+
+@pytest.mark.parametrize("column", _migratable_columns())
+def test_column_renders_as_ddl(column):
+    ddl = db._column_ddl(column)
+
+    assert ddl.startswith(column.name)
+    # MySQL refuses AUTO_INCREMENT together with the DEFAULT this helper appends
+    # to NOT NULL columns.
+    assert "AUTO_INCREMENT" not in ddl
+
+
+@pytest.mark.parametrize("column", _migratable_columns())
+def test_not_null_columns_carry_a_default(column):
+    """So ADD COLUMN cannot fail on a table that already holds rows.
+
+    Without a DEFAULT, whether the ALTER succeeds depends on the server's
+    sql_mode -- not something a deployment should hinge on.
+    """
+    if column.nullable:
+        pytest.skip("nullable column needs no seed value")
+
+    assert "DEFAULT" in db._column_ddl(column)
+
+
+def test_boolean_default_becomes_a_mysql_literal():
+    """Python's True is not a SQL keyword; it has to become 1."""
+    assert "DEFAULT 1" in _ddl(Column("flag", Boolean, nullable=False, default=True))
+
+
+def test_string_defaults_are_quoted():
+    assert "DEFAULT 'interval'" in _ddl(
+        Column("mode", String(16), nullable=False, default="interval")
+    )
+
+
+def test_integer_defaults_are_not_quoted():
+    ddl = _ddl(Column("n", Integer, nullable=False, default=0))
+
+    assert "DEFAULT 0" in ddl
+    assert "DEFAULT '0'" not in ddl
+
+
+def test_a_not_null_column_without_a_model_default_falls_back_to_zero():
+    """Belt and braces: even a column the model gives no default still migrates."""
+    ddl = _ddl(Column("n", Integer, nullable=False))
+
+    assert "DEFAULT 0" in ddl