feat: 完成图书归还前后端完整功能（管理员端）

backend/src/routes/loans.js

@@ -215,5 +215,46 @@ router.post('/lend', requireAuth, async (req, res, next) => {
     next(error);
   }
 });
+// 归还图书接口
+router.post('/return/:loanId', async (req, res) => {
+  try {
+    const { loanId } = req.params;
+
+    const loan = await prisma.loan.findUnique({
+      where: { id: parseInt(loanId) },
+      include: { book: true }
+    });
+
+    if (!loan) {
+      return res.status(404).json({ message: "借阅记录不存在" });
+    }
+    if (loan.returnedAt) {
+      return res.status(400).json({ message: "该书籍已归还" });
+    }
+
+    // 更新借阅记录为已归还
+    await prisma.loan.update({
+      where: { id: parseInt(loanId) },
+      data: {
+        returnedAt: new Date(),
+        status: "RETURNED"
+      }
+    });
 
+    // 更新书籍状态为可借阅
+    await prisma.book.update({
+      where: { id: loan.bookId },
+      data: { status: "AVAILABLE" }
+    });
+
+    res.json({
+      success: true,
+      message: "归还成功，书籍状态已更新为可借阅",
+      bookTitle: loan.book.title
+    });
+
+  } catch (error) {
+    res.status(500).json({ message: "归还失败", error: error.message });
+  }
+});
 module.exports = router;