Cooldown Bucket

The existing cooldown system tracks usage per sender (`ctx.sender` is the dict key in `cooldown_calls`). There is no way to apply a cooldown per room or globally across all users, which are common requirements for rate-limiting or noisy commands.

The idea is to add a `BucketType` that controls what the cooldown key is derived from. Per-user is the current behaviour and remains the default; per-room keys on `ctx.room.room_id`.

**Proposed API**
```python
from matrix.checks import cooldown, BucketType

@cooldown(rate=1, period=10, bucket=BucketType.room)
@bot.command()
async def announce(ctx):
await ctx.reply("Announcement!")

@cooldown(rate=5, period=60, bucket=BucketType.global_)
@bot.command()
async def search(ctx, query: str):
...
```

The change is localised to `Command.set_cooldown` in `command.py`, the `user_id = ctx.sender` line becomes a key derivation step based on the bucket type. `BucketType` can be a simple `enum.Enum`. The `cooldown` decorator in `checks.py` needs the extra parameter forwarded through; with default being global.. No structural changes to `Command` or `Registry` are needed beyond that.