schema.sql: add users.last_session

- gateway.websocket: update users.last_session

SQL for instances:
```sql
ALTER TABLE users ADD COLUMN last_session timestamp without time zone default (now() at time zone 'utc');
```

litecord/gateway/websocket.py

@@ -406,6 +406,13 @@ class GatewayWebsocket:
         # link the state to the user
         self.ext.state_manager.insert(self.state)
 
+        # update last_session
+        await self.ext.db.execute("""
+        UPDATE users
+        SET last_session = (now() at time zone 'utc')
+        WHERE id = $1
+        """, user_id)
+
         await self.update_status(presence)
         await self.subscribe_all()
         await self.dispatch_ready()

schema.sql

@@ -75,6 +75,9 @@ CREATE TABLE IF NOT EXISTS users (
     phone varchar(60) DEFAULT '',
     password_hash text NOT NULL,
 
+    -- store the last time the user logged in via the gateway
+    last_session timestamp without time zone default (now() at time zone 'utc'),
+
     PRIMARY KEY (id, username, discriminator)
 );