unique channel names (#8439)

Before this change duplicate channels were ordered arbitrarily, which
put the
collab channel in an inconsistent state.

Release Notes:

- Fixed duplicate channel names appearing in the collab sidebar.

crates/collab/migrations/20240226164505_unique_channel_names.sql

@@ -0,0 +1,3 @@
+-- Add migration script here
+
+CREATE UNIQUE INDEX uix_channels_parent_path_name ON channels(parent_path, name) WHERE (parent_path IS NOT NULL);