feat: make identity server store user instance/guild IDs

identity/migrations/20240228151314_store_guilds.sql

@@ -0,0 +1,7 @@
+create table guilds_accounts (
+    chat_instance_id text not null references chat_instances (id) on delete cascade,
+    guild_id text not null,
+    account_id text not null references accounts (id) on delete cascade,
+
+    primary key (chat_instance_id, guild_id, account_id)
+);