feat(notifications): 알림 종 + 공지 기능 추가

- 헤더 프로필 왼쪽 알림 종: 드롭다운·안읽음 배지·60초 폴링(최초 폴링 도입)
- 내 프로젝트 좋아요/즐겨찾기/댓글 시 소유자에게 알림(본인 제외·재토글 도배 방지)
- 관리자 공지: /announcements 목록 + 작성/수정/삭제, 등록 시 전 직원에게 fan-out 알림
- notifications/announcements 테이블(멱등) + requireAdmin 가드 + bell/megaphone 아이콘

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/helpers.js

@@ -29,6 +29,10 @@ const OCTICONS = {
   trash: '<path d="M11 1.75V3h2.25a.75.75 0 0 1 0 1.5H2.75a.75.75 0 0 1 0-1.5H5V1.75C5 .784 5.784 0 6.75 0h2.5C10.216 0 11 .784 11 1.75ZM4.496 6.675l.66 6.6a.25.25 0 0 0 .249.225h5.19a.25.25 0 0 0 .249-.225l.66-6.6a.75.75 0 0 1 1.492.149l-.66 6.6A1.748 1.748 0 0 1 10.595 15h-5.19a1.75 1.75 0 0 1-1.741-1.575l-.66-6.6a.75.75 0 1 1 1.492-.15ZM6.5 1.75V3h3V1.75a.25.25 0 0 0-.25-.25h-2.5a.25.25 0 0 0-.25.25Z"/>',
   book: '<path d="M0 1.75A.75.75 0 0 1 .75 1h4.253c1.227 0 2.317.59 3 1.501A3.743 3.743 0 0 1 11.006 1h4.245a.75.75 0 0 1 .75.75v10.5a.75.75 0 0 1-.75.75h-4.507a2.25 2.25 0 0 0-1.591.659l-.622.621a.75.75 0 0 1-1.06 0l-.622-.621A2.25 2.25 0 0 0 5.258 13H.75a.75.75 0 0 1-.75-.75Zm7.251 10.324.004-5.073-.002-2.253A2.25 2.25 0 0 0 5.003 2.5H1.5v9h3.757a3.75 3.75 0 0 1 1.994.574ZM8.755 4.75l-.004 7.322a3.752 3.752 0 0 1 1.992-.572H14.5v-9h-3.495a2.25 2.25 0 0 0-2.25 2.25Z"/>',
   pencil: '<path d="M11.013 1.427a1.75 1.75 0 0 1 2.474 0l1.086 1.086a1.75 1.75 0 0 1 0 2.474l-8.61 8.61c-.21.21-.47.364-.756.445l-3.251.93a.75.75 0 0 1-.927-.928l.929-3.25c.081-.286.235-.547.445-.758l8.61-8.61Zm.176 4.823L9.75 4.81l-6.286 6.287a.253.253 0 0 0-.064.108l-.558 1.953 1.953-.558a.253.253 0 0 0 .108-.064Zm1.238-3.763a.25.25 0 0 0-.354 0L10.811 3.75l1.439 1.44 1.263-1.263a.25.25 0 0 0 0-.354Z"/>',
+  // 종(알림) — Octicon bell-16
+  bell: '<path d="M8 16a2 2 0 0 0 1.985-1.75c.017-.137-.097-.25-.235-.25h-3.5c-.138 0-.252.113-.235.25A2 2 0 0 0 8 16ZM3 5a5 5 0 0 1 10 0v2.947c0 .05.015.098.042.139l1.703 2.555A1.519 1.519 0 0 1 13.482 13H2.518a1.516 1.516 0 0 1-1.263-2.36l1.703-2.554A.255.255 0 0 0 3 7.947Zm5-3.5A3.5 3.5 0 0 0 4.5 5v2.947c0 .346-.102.683-.294.97l-1.703 2.556a.017.017 0 0 0-.003.01l.001.006c0 .002.002.004.004.006l.006.004.007.001h10.964l.007-.001.006-.004.004-.006.001-.007a.017.017 0 0 0-.003-.01l-1.703-2.554a1.745 1.745 0 0 1-.294-.97V5A3.5 3.5 0 0 0 8 1.5Z"/>',
+  // 확성기(공지) — Octicon megaphone-16
+  megaphone: '<path d="M3.75 8H2.5A1.5 1.5 0 0 1 1 6.5v-1A1.5 1.5 0 0 1 2.5 4h1.25l6.767-2.71a.75.75 0 0 1 1.03.696v8.028a.75.75 0 0 1-1.03.696L3.75 8Zm-1.25-2.5v1c0 .009.007.016.016.016H3.5V4.484h-.984A.016.016 0 0 0 2.5 4.5Zm2.5.06v.88l5.75 2.302V3.258L5 5.56ZM4.5 9.25a.75.75 0 0 1 .728.568l.542 2.17c.048.19.09.38.09.512 0 .8-.649 1.5-1.5 1.5h-.375A1.5 1.5 0 0 1 3 12.5c0-.283.036-.55.09-.762l.542-2.17a.75.75 0 0 1 .728-.568h.14Z"/>',
 };
 
 // 인라인 SVG 아이콘 문자열. fill 기본은 currentColor(부모 색 상속).