refactor(题单): 删掉不再用的难度、状态、分数、提示、总分、百分比几列和 problemset_submission 表

退役第二步（第一步 0027 已在服务器和机房上线）。破坏性迁移，部署要手工放行。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/src/db/0028_drop_problemset_leftovers.sql

@@ -0,0 +1,9 @@
+-- 题单退役的第二步（第一步见 0027）：服务器和机房都已经换上不读写这几列的代码，真正删掉。
+-- 破坏性迁移，部署要手工放行（workflow_dispatch 勾 allow_destructive，或 OJ2_ALLOW_DESTRUCTIVE=1）。
+DROP TABLE "problemset_submission" CASCADE;--> statement-breakpoint
+ALTER TABLE "problemset" DROP COLUMN "difficulty";--> statement-breakpoint
+ALTER TABLE "problemset" DROP COLUMN "status";--> statement-breakpoint
+ALTER TABLE "problemset_problem" DROP COLUMN "score";--> statement-breakpoint
+ALTER TABLE "problemset_problem" DROP COLUMN "hint";--> statement-breakpoint
+ALTER TABLE "problemset_progress" DROP COLUMN "progress_percentage";--> statement-breakpoint
+ALTER TABLE "problemset_progress" DROP COLUMN "total_score";

apps/api/src/db/meta/_journal.json

@@ -197,6 +197,13 @@
       "when": 1791554831341,
       "tag": "0027_problemset_retire_columns",
       "breakpoints": true
+    },
+    {
+      "idx": 28,
+      "version": "7",
+      "when": 1791555181872,
+      "tag": "0028_drop_problemset_leftovers",
+      "breakpoints": true
     }
   ]
 }