fix tests and change invitation mechanics

database/schema.sql

@@ -90,3 +90,8 @@ CREATE TABLE IF NOT EXISTS collaboration_members (
   role TEXT NOT NULL CHECK (role IN ('owner', 'member')),
   PRIMARY KEY (group_id, session_hash)
 ) STRICT;
+
+CREATE TABLE IF NOT EXISTS collaboration_invitations (
+  group_id TEXT PRIMARY KEY NOT NULL REFERENCES collaboration_groups(id) ON DELETE CASCADE,
+  code TEXT NOT NULL
+) STRICT;