feat: back button + reopen for approved/rejected tickets

backend/src/main/java/com/srsystem/controller/TicketController.java

@@ -135,6 +135,13 @@ public class TicketController {
         return ResponseEntity.ok(ApiResponse.ok("반려되었습니다", ticketService.rejectTicket(ticketId, currentUser.getEmpId(), body.get("reason"))));
     }
 
+    @Operation(summary = "티켓 재오픈", description = "APPROVED/REJECTED 상태를 PENDING_APPROVAL로 되돌립니다")
+    @PreAuthorize("hasAnyRole('MANAGER', 'ADMIN')")
+    @PostMapping("/{ticketId}/reopen")
+    public ResponseEntity<ApiResponse<IssueTicket>> reopenTicket(@PathVariable Long ticketId) {
+        return ResponseEntity.ok(ApiResponse.ok("재오픈되었습니다", ticketService.reopenTicket(ticketId)));
+    }
+
     @Operation(summary = "댓글 목록 조회", description = "티켓의 댓글 목록을 조회합니다")
     @PreAuthorize("hasAnyRole('EMPLOYEE', 'MANAGER', 'ADMIN')")
     @GetMapping("/{ticketId}/comments")

backend/src/main/java/com/srsystem/service/TicketService.java

@@ -234,6 +234,19 @@ public class TicketService {
         return toIssueTicket(ticketRepository.save(ticket));
     }
 
+    @Transactional
+    public IssueTicket reopenTicket(Long ticketId) {
+        Ticket ticket = ticketRepository.findById(ticketId)
+                .orElseThrow(() -> new BusinessException(ErrorCode.TICKET_NOT_FOUND));
+        if (ticket.getStatus() != TicketStatus.APPROVED && ticket.getStatus() != TicketStatus.REJECTED) {
+            throw new BusinessException(ErrorCode.INVALID_STATUS);
+        }
+        ticket.setStatus(TicketStatus.PENDING_APPROVAL);
+        ticket.setApprovedBy(null);
+        ticket.setApprovedAt(null);
+        return toIssueTicket(ticketRepository.save(ticket));
+    }
+
     private IssueTicket toIssueTicket(Ticket ticket) {
         return new IssueTicket(
                 ticket.getTicketId(),