fix: eliminate phantom notifications across the entire read-state pipeline

Root cause: own messages echoed by the server marked channels unread when
the user had already navigated away. Seven related bugs compounded the
problem — stale read states, missing cleanup on space/DM removal, REST
broadcast ignoring VIEW_CHANNEL, and no validation on channel_ack writes.

Frontend:
- Skip markChannelUnread for the user's own messages (federation-aware)
- Walk backward past temp_ IDs in ackChannel instead of bailing
- Re-fire ack timer when temp message is replaced by server-confirmed ID
- Add removeChannelStates to clean up unread/read/message caches
- Clean up chatStore on removeSpace, removeDmChannel, removeInstanceSpaces

Server:
- Use sendToChannel instead of sendToSpace for REST message creation
- Clean up read_states on space deletion, member kick/leave, and ban
- Validate channel membership before accepting channel_ack writes
- Clean up read_states on DM leave and DM channel deletion

packages/web/src/components/chat/MessageList.tsx

@@ -69,6 +69,9 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
     }
   }, [channelId, loadMessages, canReadHistory]);
 
+  // Track the last message ID so the ack re-fires when a temp message is replaced by its server-confirmed ID
+  const lastMessageId = messages.length > 0 ? messages[messages.length - 1]?.id ?? '' : '';
+
   // Ack channel when messages load or when new messages arrive while near bottom
   useEffect(() => {
     if (messages.length > 0 && isNearBottom) {
@@ -76,7 +79,7 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
       ackTimerRef.current = setTimeout(() => ackChannel(channelId), 200);
     }
     return () => clearTimeout(ackTimerRef.current);
-  }, [channelId, messages.length, isNearBottom, ackChannel]);
+  }, [channelId, messages.length, lastMessageId, isNearBottom, ackChannel]);
 
   // Reset scroll tracking on channel switch so initial-load scroll fires
   useEffect(() => {