perf(server): paginate session history (#17134)

Author: nexxeln

packages/opencode/migration/20260312043431_session_message_cursor/migration.sql

@@ -0,0 +1,4 @@
+DROP INDEX IF EXISTS `message_session_idx`;--> statement-breakpoint
+DROP INDEX IF EXISTS `part_message_idx`;--> statement-breakpoint
+CREATE INDEX `message_session_time_created_id_idx` ON `message` (`session_id`,`time_created`,`id`);--> statement-breakpoint
+CREATE INDEX `part_message_id_id_idx` ON `part` (`message_id`,`id`);