feat: 公告系统 — Phase 2 完成

- Model/Repo/Service/Controller 全栈实现
- CRUD + ListActive（生效时间范围过滤）
- 管理后台独立页面 /admin/announcements（列表+弹窗CRUD）
- 前台首页 Hero 下方公告栏（info/success/warning/error 四色）
- Moderator+ 权限可管理公告

templates/MetaLab-2026/html/home/index.html

@@ -17,6 +17,18 @@
     </div>
 </section>
 
+{{if .Announcements}}
+<!-- Announcement Bar -->
+<div class="announcement-bar">
+    {{range .Announcements}}
+    <div class="announcement-item announcement-{{.Type}}">
+        <span class="announcement-title">{{.Title}}</span>
+        <span class="announcement-content">{{.Content}}</span>
+    </div>
+    {{end}}
+</div>
+{{end}}
+
 <!-- Main Content -->
 <div class="container home-main">
     <div class="home-grid">

templates/MetaLab-2026/static/css/home.css

@@ -368,6 +368,61 @@
     flex-shrink: 0;
 }
 
+/* --- Announcement Bar --- */
+.announcement-bar {
+    max-width: 900px;
+    margin: 0 auto;
+    padding: 0 16px;
+    display: flex;
+    flex-direction: column;
+    gap: 6px;
+    margin-bottom: 1.5rem;
+}
+
+.announcement-item {
+    display: flex;
+    align-items: center;
+    gap: 10px;
+    padding: 10px 16px;
+    border-radius: 8px;
+    font-size: 14px;
+    border-left: 4px solid;
+}
+
+.announcement-title {
+    font-weight: 650;
+    white-space: nowrap;
+}
+
+.announcement-content {
+    color: #374151;
+    flex: 1;
+}
+
+.announcement-info {
+    background: #dbeafe;
+    border-color: #3b82f6;
+}
+.announcement-info .announcement-title { color: #1d4ed8; }
+
+.announcement-success {
+    background: #dcfce7;
+    border-color: #22c55e;
+}
+.announcement-success .announcement-title { color: #15803d; }
+
+.announcement-warning {
+    background: #fef3c7;
+    border-color: #f59e0b;
+}
+.announcement-warning .announcement-title { color: #b45309; }
+
+.announcement-error {
+    background: #fee2e2;
+    border-color: #ef4444;
+}
+.announcement-error .announcement-title { color: #dc2626; }
+
 /* ---------- Responsive ---------- */
 @media (max-width: 960px) {
     .home-grid {