feat(api): 原子模型分组树 / 卡片 / catalog2

- 新增 /api/catalog2：多层分组树（继承格式、带条目）+ 卡片勾选列表
- 新增 /api/nodes CRUD：任意层级分组、移动（防环）、排序、根分组格式
- 新增 /api/cards CRUD 与 PUT /api/cards/{id}/groups：一张卡可混合格式、可复用分组
- 条目允许挂在任意层级分组
- 测试 59 passed；API 冒烟通过（三级分组、卡片混合格式）

src/db/mod.rs

@@ -9,9 +9,11 @@
 //!    启动时会重新建表并灌入内嵌种子。
 
 pub mod auth;
+pub mod cards;
 pub mod categories;
 mod catalog;
 pub mod entries;
+pub mod groups;
 mod schema;
 pub mod settings;
 
@@ -250,6 +252,58 @@ impl Db {
         categories::remove_group(&conn, id)
     }
 
+    // ---------- 原子模型：分组树 / 卡片 ----------
+
+    /// 完整分组树（多层，带条目）
+    pub fn load_tree(&self) -> Result<Value, StashError> {
+        let conn = self.lock()?;
+        catalog::load_tree(&conn)
+    }
+
+    pub fn load_cards(&self) -> Result<Vec<Value>, StashError> {
+        let conn = self.lock()?;
+        cards::load(&conn)
+    }
+
+    pub fn create_node(&self, new: groups::NewNode) -> Result<(), StashError> {
+        let mut conn = self.lock()?;
+        groups::create(&mut conn, new)
+    }
+
+    pub fn update_node(&self, key: &str, patch: groups::UpdateNode) -> Result<(), StashError> {
+        let mut conn = self.lock()?;
+        groups::update(&mut conn, key, patch)
+    }
+
+    pub fn remove_node(&self, key: &str) -> Result<(), StashError> {
+        let conn = self.lock()?;
+        groups::remove(&conn, key)
+    }
+
+    pub fn create_card(&self, new: cards::NewCard) -> Result<i64, StashError> {
+        let mut conn = self.lock()?;
+        cards::create(&mut conn, new)
+    }
+
+    pub fn update_card(&self, id: i64, patch: cards::UpdateCard) -> Result<(), StashError> {
+        let mut conn = self.lock()?;
+        cards::update(&mut conn, id, patch)
+    }
+
+    pub fn remove_card(&self, id: i64) -> Result<(), StashError> {
+        let conn = self.lock()?;
+        cards::remove(&conn, id)
+    }
+
+    pub fn set_card_groups(
+        &self,
+        id: i64,
+        groups: Vec<cards::CardGroupInput>,
+    ) -> Result<(), StashError> {
+        let mut conn = self.lock()?;
+        cards::set_groups(&mut conn, id, groups)
+    }
+
     pub fn remove_category(&self, id: &str) -> Result<(), StashError> {
         let mut conn = self.lock()?;
         categories::remove(&mut conn, id)