UI 개선 — 신고 목록 표에 발생일시 컬럼 추가

신고일시 앞에 발생일시(occurred_at) 컬럼 추가.
미입력 시 '-' 표시. 관리자·옵저버 신고 목록·대시보드 모두 반영.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/static/pages/observer/dashboard.html

@@ -56,7 +56,7 @@
         <table>
           <thead><tr>
             <th>접수번호</th><th>충전기ID</th><th>충전소명</th><th>문제유형</th>
-            <th>에러코드</th><th>신고일시</th><th>정비사</th><th>상태</th>
+            <th>에러코드</th><th>발생일시</th><th>신고일시</th><th>정비사</th><th>상태</th>
           </tr></thead>
           <tbody id="tbody"></tbody>
         </table>
@@ -112,6 +112,7 @@ function render(chargerId) {
       <td>${r.station_name || '-'}</td>
       <td>${(r.issue_types || []).join(', ') || '-'}</td>
       <td>${r.error_code || '-'}</td>
+      <td>${r.occurred_at ? Auth.fmtDt(r.occurred_at) : '<span style="color:var(--gray4)">-</span>'}</td>
       <td>${Auth.fmtDt(r.reported_at)}</td>
       <td>${r.mechanic_name || '-'}</td>
       <td>${Auth.statusBadge(r.status)}</td>