Fix db inconsistencies when deleting a room

Since the euph_trees table can't have any foreign key constraints
pointing to the euph_rooms table, deleting a room wouldn't delete that
room's trees in euph_trees. Upon reconnecting to the room, those trees
would then be displayed as placeholder messages without children.

src/vault/euph.rs

@@ -472,13 +472,25 @@ impl EuphRequest {
     }
 
     fn delete(conn: &mut Connection, room: String) -> rusqlite::Result<()> {
-        conn.execute(
+        let tx = conn.transaction()?;
+
+        tx.execute(
             "
             DELETE FROM euph_rooms
             WHERE room = ?
             ",
-            [room],
+            [&room],
         )?;
+
+        tx.execute(
+            "
+            DELETE FROM euph_trees
+            WHERE room = ?
+            ",
+            [&room],
+        )?;
+
+        tx.commit()?;
         Ok(())
     }
 

src/vault/prepare.rs

@@ -4,7 +4,6 @@ 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 (