fix(cron): 分离调度器状态与数据库同步状态，修复持久化任务在提前启动场景下未加载的问题

[gomico]

修复了插件在核心启动前通过 add_basic_job(enabled=True) 注册 BasicCronJob 时，导致 CronJobManager.start() 跳过 sync_from_db()，数据库中已启用的持久化 CronJob 未加载到 APScheduler 的问题。

根因：_started 变量同时兼任两个职责——"APScheduler 是否已运行"和"sync_from_db() 是否已执行"。插件在 plugin_manager.reload() 阶段调用 add_basic_job() 时，_schedule_job() 会提前启动 scheduler 并置 _started = True；随后核心生命周期调用 start() 看到 _started 为真直接返回，sync_from_db() 被永久跳过。

关联 issue： #9418

Modifications / 改动点

• astrbot/core/cron/manager.py：引入独立状态 _db_synced，将"APScheduler 运行状态"与"数据库同步状态"解耦。

  • start() 的提前返回条件改为 _db_synced，而非 _started。
  • scheduler 启动仍受 _started 保护，避免重复调用 scheduler.start()。
  • sync_from_db() 完成后才置 _db_synced = True，确保无论 scheduler 是否被提前启动，数据库中的持久化任务都能被加载。

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

修复前（上次重启）：cost_control 注册 CronJob → _started=True → start() 直接 return → sync_from_db() 未执行，所有持久化 job 的 updated_at 未被更新，原有定时任务静默错过。

修复后（本次重启验证）：

日志序列：
  01:37:40  [cost_control] CronJob 注册完成          ← 插件 init 调用 add_basic_job，_started=True
  01:37:53  AstrBot started.                         ← start() 执行，_started 已为 True 但 _db_synced=False

数据库验证（updated_at 全部更新到 01:37:53，确认 sync_from_db 已执行）：
  [一次性测试任务]            ✓ updated 17:37:53
  [定时提醒任务A]             ✓ updated 17:37:53  ← 之前卡住未恢复的任务
  [定时提醒任务B]             ✓ updated 17:37:53
  [定时提醒任务C]             ✓ updated 17:37:53
  [定时天气播报任务]           ✓ updated 17:37:53
  [定时提醒任务D]             ✓ updated 17:37:53
  [定时提醒任务E]             ✓ updated 17:37:53
  [定时提醒任务F]             ✓ updated 17:37:53
  [一次性提醒任务]            ✓ updated 17:37:53
  [已禁用的语音播报任务]       ✗ disabled，被跳过

（时间戳为 UTC，日志为 CST；数据库使用 sqlite3 直查确认。）

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Decouple the cron scheduler running state from the database sync state to ensure persistent jobs are always loaded, even when the scheduler is started early.

Bug Fixes:

• Ensure database-synced persistent cron jobs are loaded into the scheduler when basic jobs start the scheduler before the core start sequence.

Tests:

• Add a regression test verifying that early scheduler startup does not cause the cron manager to skip syncing jobs from the database.

[sourcery-ai]

Hey - I've left some high level feedback:

• Consider resetting _db_synced (e.g., in shutdown or a dedicated reset method) so that a manager reused after shutdown will correctly resync from the database rather than skipping sync_from_db() on the next start() call.
• The introduction of _db_synced changes the state machine for CronJobManager; adding a brief comment or diagram of the expected _started/_db_synced transitions near start() would help future maintainers reason about edge cases like early starts and restarts.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider resetting `_db_synced` (e.g., in `shutdown` or a dedicated reset method) so that a manager reused after shutdown will correctly resync from the database rather than skipping `sync_from_db()` on the next `start()` call.
- The introduction of `_db_synced` changes the state machine for `CronJobManager`; adding a brief comment or diagram of the expected `_started`/`_db_synced` transitions near `start()` would help future maintainers reason about edge cases like early starts and restarts.

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