Replace i32 with u32 for database columns

We never expect to return signed integers and so we shouldn't use
a signed type. I think this was a limitation of sqlx.

crates/collab/src/db/project.rs

@@ -8,7 +8,7 @@ pub struct Model {
     pub id: ProjectId,
     pub room_id: RoomId,
     pub host_user_id: UserId,
-    pub host_connection_id: i32,
+    pub host_connection_id: u32,
 }
 
 #[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]