feat(admin): 后台新增条目与分组 CRUD

- POST /api/items + DELETE /api/items/{key}：新增应用/海报/图文，按分区/分组挂载；PATCH 支持 nodeKey 移动
- 分组 CRUD：POST /api/categories/{id}/groups、PATCH/DELETE /api/groups/{id}（删除级联组内条目）
- PATCH /api/categories/{id} 增加 blockNodeKey，首页卡片可选择预览分组
- /admin 新增「内容」Tab（新增/删除条目），「分区」Tab 内联分组管理与卡片预览
- 测试 59 passed（条目创建/移动/删除、分组增删改）

src/db/mod.rs

@@ -185,6 +185,16 @@ impl Db {
         entries::load(&conn, key)
     }
 
+    pub fn create_entry(&self, new: entries::NewEntry) -> Result<EntryRecord, StashError> {
+        let mut conn = self.lock()?;
+        entries::create(&mut conn, new)
+    }
+
+    pub fn remove_entry(&self, key: &str) -> Result<(), StashError> {
+        let conn = self.lock()?;
+        entries::remove(&conn, key)
+    }
+
     pub fn update_entry(
         &self,
         key: &str,
@@ -221,6 +231,25 @@ impl Db {
         categories::update(&mut conn, id, patch)
     }
 
+    pub fn create_group(
+        &self,
+        section_id: &str,
+        new: categories::NewGroup,
+    ) -> Result<(), StashError> {
+        let mut conn = self.lock()?;
+        categories::create_group(&mut conn, section_id, new)
+    }
+
+    pub fn update_group(&self, id: &str, patch: categories::UpdateGroup) -> Result<(), StashError> {
+        let conn = self.lock()?;
+        categories::update_group(&conn, id, patch)
+    }
+
+    pub fn remove_group(&self, id: &str) -> Result<(), StashError> {
+        let conn = self.lock()?;
+        categories::remove_group(&conn, id)
+    }
+
     pub fn remove_category(&self, id: &str) -> Result<(), StashError> {
         let mut conn = self.lock()?;
         categories::remove(&mut conn, id)