feat: 新增 rebuild_sql_display 管理命令，为存量 SQL 题回填展示数据

生成逻辑从 ProblemBase._build_sql_display 提取为 problem.utils.generate_sql_display，
管理端保存与回填命令共用。命令支持 --dry-run / --force。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

problem/management/commands/rebuild_sql_display.py

@@ -0,0 +1,36 @@
+from django.core.management.base import BaseCommand
+
+from problem.models import Problem
+from problem.utils import generate_sql_display
+
+
+class Command(BaseCommand):
+    help = "为存量 SQL 题目重新生成 sql_display 展示数据（新建/编辑题目时会自动生成，此命令用于回填老题）"
+
+    def add_arguments(self, parser):
+        parser.add_argument("--dry-run", action="store_true", help="只检查并打印结果，不写数据库")
+        parser.add_argument("--force", action="store_true", help="已有 sql_display 的题目也重新生成（默认跳过）")
+
+    def handle(self, *args, **options):
+        qs = Problem.objects.filter(sql_config__isnull=False).order_by("id")
+        ok = failed = skipped = 0
+        for p in qs:
+            label = f"id={p.id} _id={p._id}" + (f" contest={p.contest_id}" if p.contest_id else "") + f" {p.title}"
+            if p.sql_display is not None and not options["force"]:
+                skipped += 1
+                self.stdout.write(f"SKIP  {label}（已有 sql_display，--force 可强制重建）")
+                continue
+            display, error = generate_sql_display(p.test_case_id, p.answers, p.sql_config)
+            if error:
+                failed += 1
+                self.stderr.write(self.style.ERROR(f"FAIL  {label}: {error}"))
+                continue
+            if not options["dry_run"]:
+                p.sql_display = display
+                p.save(update_fields=["sql_display"])
+            ok += 1
+            self.stdout.write(self.style.SUCCESS(f"OK    {label}"))
+        summary = f"完成：成功 {ok}，失败 {failed}，跳过 {skipped}，共 {qs.count()} 道 SQL 题"
+        if options["dry_run"]:
+            summary += "（dry-run，未写库）"
+        self.stdout.write(summary)