feat(diary): 添加发现页 Discover API — 每日灵感/热门标签/精选模板/专家日记

新增 DiscoverService 并发聚合 4 个数据区:
- daily_inspiration: MD5 哈希确定性日更推荐，匿名作者名
- hot_topics: 标签频率统计 Top 8
- featured_templates: 官方模板最多 6 个
- expert_diaries: 评论数热度排序，去重最多 5 位作者

GET /api/v1/diary/discover + utoipa 文档 + diary.journal.read 权限守卫

crates/erp-server/src/main.rs

@@ -208,6 +208,7 @@ struct MessageApiDoc;
         erp_diary::handler::parent_handler::list_pending_bindings,
         erp_diary::handler::parent_handler::confirm_binding,
         erp_diary::handler::parent_handler::reject_binding,
+        erp_diary::handler::discover_handler::get_discover,
     ),
     components(schemas(
         erp_diary::dto::CreateJournalReq,
@@ -241,6 +242,10 @@ struct MessageApiDoc;
         erp_diary::handler::parent_handler::DeleteChildDataReq,
         erp_diary::handler::parent_handler::BindingResp,
         erp_diary::handler::parent_handler::DeleteResultResp,
+        erp_diary::dto::DiscoverResp,
+        erp_diary::dto::InspirationItem,
+        erp_diary::dto::TagCount,
+        erp_diary::dto::ExpertDiaryItem,
     ))
 )]
 struct DiaryApiDoc;