Scheduler: Add list_jobs method

There is currently no way to inspect which jobs are registered at runtime. list_jobs() returns their names, which is useful for admin commands or debugging.

### Proposed API
```python
def list_jobs(self) -> list[str]
```

### Example
```python
@bot.command()
async def jobs(ctx: Context):
await ctx.reply("\n".join(bot.scheduler.list_jobs()))
```

### Before opening a PR
- Write unit tests
- Run mypy matrix/
- Run pytest tests/
- Run black .