Store room member read markers in model

src/backend/models/items.py

@@ -158,6 +158,8 @@ class Member(ModelItem):
     power_level:     int      = 0
     invited:         bool     = False
     profile_updated: datetime = ZeroDate
+    last_read_event: str      = ""
+    last_read_at:    datetime = ZeroDate
 
     def __lt__(self, other: "Member") -> bool:
         """Sort by power level, then by display name/user ID."""

src/backend/nio_callbacks.py

@@ -537,3 +537,20 @@ class NioCallbacks:
             room.user_name(user_id) or user_id for user_id in ev.users
             if user_id not in self.client.backend.clients
         )
+
+
+    async def onReceiptEvent(
+        self, room: nio.MatrixRoom, ev: nio.ReceiptEvent,
+    ) -> None:
+        model = self.models[self.user_id, room.room_id, "members"]
+
+        for receipt in ev.receipts:
+            if receipt.receipt_type != "m.read":
+                continue
+
+            member = model.get(receipt.user_id)
+
+            if member:
+                member.last_read_event = receipt.event_id
+                timestamp              = receipt.timestamp / 1000
+                member.last_read_at    = datetime.fromtimestamp(timestamp)