feat: step artifacts framework

- Add Artifact type to Step (name, path, artifact_type, description)
- step_done tool accepts optional artifacts parameter
- Save artifacts to step_artifacts DB table
- Display artifacts in frontend PlanSection (tag style)
- Show artifacts in step context for sub-agents and coordinator
- Add LLM client retry with exponential backoff

src/db.rs

@@ -59,6 +59,13 @@ impl Database {
         .execute(&self.pool)
         .await;
 
+        // Migration: add template_id column to workflows
+        let _ = sqlx::query(
+            "ALTER TABLE workflows ADD COLUMN template_id TEXT NOT NULL DEFAULT ''"
+        )
+        .execute(&self.pool)
+        .await;
+
         // Migration: add deleted column to projects
         let _ = sqlx::query(
             "ALTER TABLE projects ADD COLUMN deleted INTEGER NOT NULL DEFAULT 0"
@@ -208,6 +215,20 @@ impl Database {
         .execute(&self.pool)
         .await?;
 
+        sqlx::query(
+            "CREATE TABLE IF NOT EXISTS step_artifacts (
+                id TEXT PRIMARY KEY,
+                workflow_id TEXT NOT NULL,
+                step_order INTEGER NOT NULL,
+                name TEXT NOT NULL,
+                path TEXT NOT NULL,
+                artifact_type TEXT NOT NULL DEFAULT 'file',
+                description TEXT NOT NULL DEFAULT ''
+            )"
+        )
+        .execute(&self.pool)
+        .await?;
+
         Ok(())
     }
 }
@@ -231,6 +252,7 @@ pub struct Workflow {
     pub status: String,
     pub created_at: String,
     pub report: String,
+    pub template_id: String,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, sqlx::FromRow)]