feat(backup): say whether an account can take an erased account's copy

A restore replaces the account's records and has no merge, so the copy
of an account that no longer exists may only go into an account that
holds none of the records a backup carries. The check reads every
backed-up model by its userId, reaches the owner-less children through
their parent, and is held to the schema so a new model cannot be read
as empty. Managed profiles and accounts pending deletion are refused as
well. The admin console asks GET /api/admin/backups/targets/{userId}
before it sends a file again, so a refusal costs no upload.

src/app/api/admin/backups/targets/[userId]/route.ts

@@ -0,0 +1,59 @@
+/**
+ * GET /api/admin/backups/targets/{userId} — whether the copy of an erased
+ * account may be restored into this account.
+ *
+ * The backups console asks before it sends a file a second time with that
+ * account as its target, so a refusal costs no upload. The upload decides
+ * again on its own (`checkRestoreTarget` in `restore-target.ts`); this answer
+ * is advice, not a reservation. It says no for a managed profile, an account
+ * pending deletion, and an account that already holds records, naming the
+ * models that hold them. Counts and record content are not read.
+ */
+import { NextRequest } from "next/server";
+import { z } from "zod/v4";
+
+import { apiHandler, HttpError, requireAdmin } from "@/lib/api-handler";
+import { apiSuccess, returnAllZodIssues } from "@/lib/api-response";
+import { prisma } from "@/lib/db";
+import { checkRestoreTarget } from "@/lib/export/restore-target";
+import { annotate } from "@/lib/logging/context";
+import { checkRateLimit } from "@/lib/rate-limit";
+
+export const dynamic = "force-dynamic";
+
+const paramsSchema = z.object({ userId: z.string().min(1).max(64) });
+
+export const GET = apiHandler(
+  async (
+    _request: NextRequest,
+    { params }: { params: Promise<{ userId: string }> },
+  ) => {
+    const { user: admin } = await requireAdmin();
+    annotate({ action: { name: "admin.backups.target.check" } });
+
+    const rl = await checkRateLimit(
+      `admin-backups-target:${admin.id}`,
+      30,
+      60 * 1000,
+    );
+    if (!rl.allowed) {
+      throw new HttpError(429, "Too many account checks");
+    }
+
+    const parsed = paramsSchema.safeParse(await params);
+    if (!parsed.success) return returnAllZodIssues(parsed.error);
+
+    const verdict = await checkRestoreTarget(prisma, parsed.data.userId);
+    if (!verdict.ok && verdict.reason === "not_found") {
+      throw new HttpError(404, "User not found");
+    }
+    const target = verdict.target!;
+    return apiSuccess({
+      id: target.id,
+      username: target.username,
+      eligible: verdict.ok,
+      reason: verdict.ok ? null : verdict.reason,
+      models: verdict.ok ? [] : verdict.models,
+    });
+  },
+);