Add euph_trees todo

src/vault/prepare.rs

@@ -2,6 +2,9 @@ use rusqlite::Connection;
 
 pub fn prepare(conn: &mut Connection) -> rusqlite::Result<()> {
     println!("Opening vault");
+    // This temporary table has no foreign key constraint on euph_rooms since
+    // cross-schema constraints like that are not supported by SQLite.
+    // TODO Remove entries from this table whenever a room is deleted
     conn.execute_batch(
         "
         CREATE TEMPORARY TABLE euph_trees (