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/admin/static/css/announcements.css

@@ -0,0 +1,162 @@
+/* ============================================================
+   Announcements Admin — 公告管理页面样式
+   ============================================================ */
+
+/* Badge 类型色 */
+.badge-info { background: #dbeafe; color: #1d4ed8; }
+.badge-success { background: #dcfce7; color: #15803d; }
+.badge-warning { background: #fef3c7; color: #b45309; }
+.badge-error { background: #fee2e2; color: #dc2626; }
+
+.badge-active { background: #dcfce7; color: #15803d; }
+.badge-inactive { background: #f3f4f6; color: #6b7280; }
+
+.badge {
+    display: inline-block;
+    padding: 2px 8px;
+    border-radius: 4px;
+    font-size: 12px;
+    font-weight: 500;
+}
+
+/* Toolbar */
+.toolbar {
+    margin-bottom: 16px;
+}
+
+/* Data table */
+.data-table {
+    width: 100%;
+    border-collapse: collapse;
+    background: #fff;
+    border-radius: 8px;
+    overflow: hidden;
+    box-shadow: 0 1px 3px rgba(0,0,0,0.1);
+}
+
+.data-table th,
+.data-table td {
+    padding: 12px 16px;
+    text-align: left;
+    border-bottom: 1px solid #e5e7eb;
+    font-size: 14px;
+}
+
+.data-table th {
+    background: #f9fafb;
+    color: #6b7280;
+    font-weight: 600;
+    font-size: 12px;
+    text-transform: uppercase;
+}
+
+.data-table tbody tr:hover {
+    background: #f9fafb;
+}
+
+.table-empty {
+    text-align: center;
+    color: #9ca3af;
+    padding: 40px 0;
+}
+
+.btn-edit {
+    color: #3b82f6;
+    text-decoration: none;
+    margin-right: 12px;
+}
+
+.btn-delete {
+    color: #ef4444;
+    text-decoration: none;
+}
+
+/* Modal 表单 */
+.form-row {
+    display: flex;
+    gap: 16px;
+}
+
+.form-row .form-group {
+    flex: 1;
+}
+
+.form-group {
+    margin-bottom: 16px;
+}
+
+.form-group label {
+    display: block;
+    margin-bottom: 6px;
+    font-size: 14px;
+    font-weight: 500;
+    color: #374151;
+}
+
+.form-input,
+.form-textarea,
+.form-select {
+    width: 100%;
+    padding: 8px 12px;
+    border: 1px solid #d1d5db;
+    border-radius: 6px;
+    font-size: 14px;
+    box-sizing: border-box;
+}
+
+.form-textarea {
+    resize: vertical;
+}
+
+.checkbox-label {
+    display: flex !important;
+    align-items: center;
+    gap: 8px;
+    cursor: pointer;
+}
+
+.checkbox-label input[type="checkbox"] {
+    width: 16px;
+    height: 16px;
+}
+
+/* Pagination */
+.pagination {
+    margin-top: 16px;
+    display: flex;
+    gap: 4px;
+    justify-content: center;
+}
+
+.page-item {
+    display: inline-block;
+    padding: 6px 12px;
+    border: 1px solid #d1d5db;
+    border-radius: 4px;
+    color: #374151;
+    text-decoration: none;
+    font-size: 14px;
+}
+
+.page-item.active {
+    background: #3b82f6;
+    color: #fff;
+    border-color: #3b82f6;
+}
+
+/* Toast */
+.toast {
+    display: none;
+    position: fixed;
+    bottom: 24px;
+    left: 50%;
+    transform: translateX(-50%);
+    padding: 10px 24px;
+    border-radius: 8px;
+    color: #fff;
+    font-size: 14px;
+    z-index: 9999;
+}
+
+.toast-success { background: #22c55e; }
+.toast-error { background: #ef4444; }