feat(notifications): add leave, kick and ban notifications with actor pills

Extend the notification system to include member leave, kick and ban events,
mirroring the existing join notification functionality.

### Changes
- Added `_handle_leave_notifications` to process LEAVE, KICK and BAN events
- Integrated new notifications into existing membership event handlers
- Introduced `{actor}` placeholder with Matrix URI pill support
- Added `{actor_display}` as plaintext fallback
- Extended template renderer to support actor context
- Ensured consistent rendering across plaintext and HTML messages
- Added template cleanup for cases without an actor (e.g. voluntary leave)

### Templates
New placeholders:
- `{actor}` → clickable Matrix user pill
- `{actor_display}` → plain display name

### Configuration
New config keys:
- `leave_notification_message`
- `kick_notification_message`
- `ban_notification_message`

### Notes
- Fully async and non-blocking implementation
- Backwards compatible with existing templates
- No impact on join notification logic
- Scoped to rooms within configured space

base-config.yaml

@@ -91,7 +91,7 @@ welcome_sleep: 0
 # (optional)
 notification_room: 
 
-# message to send to the notification room when someone joins one of the above rooms:
+# message to send to the notification room when someone joins or leave one of the above rooms:
 # available placeholders:
 # - {user}: display name of the joining user (falls back to localpart or user ID)
 # - {user_id}: full Matrix user ID
@@ -101,6 +101,15 @@ notification_room:
 # - {room_id}: raw room ID
 join_notification_message: |
   {user} has joined {room}.
+  
+leave_notification_message: |
+  {user} has left {room}.
+  
+kick_notification_message: |
+  {user} was kicked by {actor} from {room}.
+  
+ban_notification_message: |
+  {user} was baned by {actor} from {room}.
 
 # whether to censor files/messages
 # can be boolean (true/false) for all-or-nothing behavior,