chore: implement database crud for AI seat usage (#22681)

Creates a new table `ai_seat_state` to keep track of when users consume an ai_seat. Once a user consumes an AI seat, they will forever in this table (as it stands today).

coderd/database/migrations/testdata/fixtures/000439_ai_seat_state.up.sql

@@ -0,0 +1,11 @@
+INSERT INTO
+	ai_seat_state (
+		user_id,
+		first_used_at,
+		last_used_at,
+		last_event_type,
+		last_event_description,
+		updated_at
+	)
+VALUES
+	('30095c71-380b-457a-8995-97b8ee6e5307', NOW(), NOW(), 'task'::ai_seat_usage_reason, 'Used for AI task', NOW());