优化: 任务列表后端真分页(limit/offset 下推 + count 查询)

- 后端新增 count_by_query 方法(复用 list_by_query 的 WHERE 逻辑)
- 前端 buildTaskQuery 传 limit/offset 到后端(SQL 下推分页)
- 去除前端客户端 slice(后端已分页,直接分组)
- Paginator total 用 totalTasks(当前用当前页条数,后续接 count IPC)

crates/df-storage/src/crud/task_repo.rs

@@ -437,6 +437,60 @@ impl TaskRepo {
         .map_err(storage_err)?
     }
 
+    /// 按 TaskQuery 条件计数(不含 limit/offset,用于分页 total)。
+    ///
+    /// 复用 list_by_query 的 WHERE 构造逻辑(仅 WHERE,无 ORDER BY/LIMIT),
+    /// 返回满足条件的总行数(忽略分页裁剪)。
+    pub async fn count_by_query(&self, query: &TaskQuery) -> Result<i64> {
+        let conn = self.conn.clone();
+        let project_id = query.project_id.clone();
+        let status = query.status.clone();
+        let priority = query.priority;
+        let keyword = query.keyword.clone();
+
+        tokio::task::spawn_blocking(move || {
+            let guard = conn.blocking_lock();
+
+            let mut where_clauses: Vec<String> = vec!["deleted_at IS NULL".to_string()];
+            let mut params_vec: Vec<Box<dyn rusqlite::ToSql>> = Vec::new();
+
+            if let Some(ref pid) = project_id {
+                where_clauses.push(format!("project_id = ?{}", params_vec.len() + 1));
+                params_vec.push(Box::new(pid.clone()));
+            }
+            if let Some(ref s) = status {
+                where_clauses.push(format!("status = ?{}", params_vec.len() + 1));
+                params_vec.push(Box::new(s.clone()));
+            }
+            if let Some(p) = priority {
+                where_clauses.push(format!("priority = ?{}", params_vec.len() + 1));
+                params_vec.push(Box::new(p));
+            }
+            if let Some(ref kw) = keyword {
+                let escaped = kw.replace('%', "\\%").replace('_', "\\_");
+                let pat = format!("%{escaped}%");
+                let p1 = params_vec.len() + 1;
+                let p2 = p1 + 1;
+                where_clauses.push(format!("(title LIKE ?{p1} OR description LIKE ?{p2}) ESCAPE '\\'"));
+                params_vec.push(Box::new(pat.clone()));
+                params_vec.push(Box::new(pat));
+            }
+
+            let sql = format!(
+                "SELECT COUNT(*) FROM tasks WHERE {}",
+                where_clauses.join(" AND ")
+            );
+            let param_refs: Vec<&dyn rusqlite::ToSql> =
+                params_vec.iter().map(|p| p.as_ref()).collect();
+            let count: i64 = guard
+                .query_row(&sql, param_refs.as_slice(), |row| row.get(0))
+                .map_err(storage_err)?;
+            Ok(count)
+        })
+        .await
+        .map_err(|e| storage_err(format!("count_by_query join failed: {e}")))?
+    }
+
     /// 查某父任务的全部子任务(parent_id = ?,deleted_at IS NULL),按创建时间升序。
     ///
     /// 知识图谱 Phase 1 V29(对标设计 §2.1 父任务聚合规则):父任务=容器模型,status