add basic guild create + message create endpoints

chat/migrations/20240116203402_init.sql

@@ -31,6 +31,8 @@ create table guilds_users (
     guild_id text not null references guilds (id) on delete cascade,
     user_id  text not null references users (id) on delete cascade,
 
+    joined_at timestamptz not null default now(),
+
     primary key (guild_id, user_id)
 );