feat(flow): 重构流程引擎与任务执行器架构

重构流程引擎核心组件，引入执行器接口Executor替代原有TaskComponent，优化节点配置映射逻辑：
1. 新增mappers模块集中处理节点配置提取
2. 为存储层添加Storage trait抽象
3. 移除对ctx魔法字段的依赖，直接传递节点信息
4. 增加构建器模式支持引擎创建
5. 完善DSL解析的输入校验

同时标记部分未使用代码为allow(dead_code)

backend/src/flow/mappers/db.rs

@@ -0,0 +1,35 @@
+use serde_json::Value;
+
+// Extract db config: sql, params, outputKey, connection from a node
+pub fn extract_db_cfg(n: &Value) -> Option<Value> {
+    let data = n.get("data");
+    let db_cfg = data.and_then(|d| d.get("db")).and_then(|v| v.as_object())?;
+
+    let mut db_obj = serde_json::Map::new();
+    // sql can be string or object with content
+    let raw_sql = db_cfg.get("sql");
+    let sql = match raw_sql {
+        Some(Value::String(s)) => super::sanitize_wrapped(s),
+        Some(Value::Object(o)) => o
+            .get("content")
+            .and_then(|v| v.as_str())
+            .map(super::sanitize_wrapped)
+            .unwrap_or_default(),
+        _ => String::new(),
+    };
+    if !sql.is_empty() {
+        db_obj.insert("sql".into(), Value::String(sql));
+    }
+
+    if let Some(p) = db_cfg.get("params") {
+        db_obj.insert("params".into(), p.clone());
+    }
+    if let Some(Value::String(k)) = db_cfg.get("outputKey") {
+        db_obj.insert("outputKey".into(), Value::String(k.clone()));
+    }
+    if let Some(conn) = db_cfg.get("connection") {
+        db_obj.insert("connection".into(), conn.clone());
+    }
+
+    if db_obj.is_empty() { None } else { Some(Value::Object(db_obj)) }
+}

backend/src/flow/mappers/http.rs

@@ -0,0 +1,65 @@
+use serde_json::Value;
+
+// Extract http config: method, url, headers, query, body from a node
+pub fn extract_http_cfg(n: &Value) -> Option<Value> {
+    let data = n.get("data");
+    let api = data.and_then(|d| d.get("api"));
+    let method = api
+        .and_then(|a| a.get("method"))
+        .and_then(|v| v.as_str())
+        .unwrap_or("GET")
+        .to_string();
+    let url_val = api.and_then(|a| a.get("url"));
+    let raw_url = match url_val {
+        Some(Value::String(s)) => s.clone(),
+        Some(Value::Object(obj)) => obj
+            .get("content")
+            .and_then(|v| v.as_str())
+            .unwrap_or("")
+            .to_string(),
+        _ => String::new(),
+    };
+    let url = super::sanitize_wrapped(&raw_url);
+    if url.is_empty() {
+        return None;
+    }
+
+    let mut http_obj = serde_json::Map::new();
+    http_obj.insert("method".into(), Value::String(method));
+    http_obj.insert("url".into(), Value::String(url));
+
+    // Optional: headers
+    if let Some(hs) = api.and_then(|a| a.get("headers")).and_then(|v| v.as_object()) {
+        let mut heads = serde_json::Map::new();
+        for (k, v) in hs.iter() {
+            if let Some(s) = v.as_str() {
+                heads.insert(k.clone(), Value::String(s.to_string()));
+            }
+        }
+        if !heads.is_empty() {
+            http_obj.insert("headers".into(), Value::Object(heads));
+        }
+    }
+
+    // Optional: query
+    if let Some(qs) = api.and_then(|a| a.get("query")).and_then(|v| v.as_object()) {
+        let mut query = serde_json::Map::new();
+        for (k, v) in qs.iter() {
+            query.insert(k.clone(), v.clone());
+        }
+        if !query.is_empty() {
+            http_obj.insert("query".into(), Value::Object(query));
+        }
+    }
+
+    // Optional: body
+    if let Some(body_obj) = data.and_then(|d| d.get("body")).and_then(|v| v.as_object()) {
+        if let Some(Value::Object(json_body)) = body_obj.get("json") {
+            http_obj.insert("body".into(), Value::Object(json_body.clone()));
+        } else if let Some(Value::String(s)) = body_obj.get("content") {
+            http_obj.insert("body".into(), Value::String(s.clone()));
+        }
+    }
+
+    Some(Value::Object(http_obj))
+}