chore: merge session last seen migration into initial

migrations/2025-02-23-095541_initial/up.sql

@@ -12,6 +12,8 @@ create table sessions (
         references users (id)
         on delete cascade,
 
+    last_seen bigint not null,
+
     unique (id, user_id)
 );
 

migrations/2025-03-15-145721_session_last_seen/down.sql

@@ -1,3 +0,0 @@
--- This file should undo anything in `up.sql`
-alter table sessions
-    drop column last_seen;

migrations/2025-03-15-145721_session_last_seen/up.sql

@@ -1,3 +0,0 @@
--- Your SQL goes here
-alter table sessions
-    add column last_seen bigint not null default 0;