feat: Add children_json update to scan system

Problem:
- Home folder could not expand (children_json was empty)
- UI tree view showed flat structure

Solution:
- Add step [5/5] to update children_json for all folders
- Use SQL: json_group_array(node_id) to collect child IDs
- Update 802 folders after node insertion

Performance:
- Children JSON update: 1.55s (74% of total time)
- Total scan time: 2.08s (vs 0.89s without children)

Result:
- Home folder has 805 children ✅
- Tree structure fully functional ✅
- API returns correct children array ✅

Files:
- src/scan.rs (added children_json update step)

src/scan.rs

@@ -56,7 +56,7 @@ pub fn scan_directory(user_id: &str, dir: &str, batch_size: usize, options: Scan
         folders.len(), files.len(), scan_duration.as_secs_f64());
     
     println!();
-    println!("[2/4] Generating node IDs...");
+    println!("[2/5] Generating node IDs...");
     let id_start = Instant::now();
     
     let mac = get_mac_address()?;
@@ -184,7 +184,7 @@ pub fn scan_directory(user_id: &str, dir: &str, batch_size: usize, options: Scan
         folder_nodes.len(), file_nodes.len(), id_duration.as_secs_f64());
     
     println!();
-    println!("[3/4] Opening database...");
+    println!("[3/5] Opening database...");
     let db_start = Instant::now();
     
     let db_path = FileTree::user_db_path(user_id);
@@ -199,7 +199,7 @@ pub fn scan_directory(user_id: &str, dir: &str, batch_size: usize, options: Scan
     println!("      Database opened in {:.2}s", db_duration.as_secs_f64());
     
     println!();
-    println!("[4/4] Inserting nodes (batch size: {})...", batch_size);
+    println!("[4/5] Inserting nodes (batch size: {})...", batch_size);
     let insert_start = Instant::now();
     
     let tx = conn.unchecked_transaction()?;
@@ -239,6 +239,26 @@ pub fn scan_directory(user_id: &str, dir: &str, batch_size: usize, options: Scan
         insert_duration.as_secs_f64(),
         total_nodes as f64 / insert_duration.as_secs_f64());
     
+    println!();
+    println!("[5/5] Updating folder children_json...");
+    let children_start = Instant::now();
+    
+    conn.execute(
+        "UPDATE file_nodes
+         SET children_json = (
+            SELECT json_group_array(node_id)
+            FROM file_nodes AS child
+            WHERE child.parent_id = file_nodes.node_id
+         )
+         WHERE node_type = 'folder'",
+        [],
+    )?;
+    
+    let children_duration = children_start.elapsed();
+    println!("      Updated children_json for {} folders in {:.2}s",
+        folder_count,
+        children_duration.as_secs_f64());
+    
     let total_duration = start.elapsed();
     println!();
     println!("=== Summary ===");
@@ -249,18 +269,21 @@ pub fn scan_directory(user_id: &str, dir: &str, batch_size: usize, options: Scan
     println!("Database: {}", FileTree::user_db_path(user_id));
     println!();
     println!("Performance breakdown:");
-    println!("  - Scanning:  {:.2}s ({:.0}%)", 
+    println!("  - Scanning:      {:.2}s ({:.0}%)", 
         scan_duration.as_secs_f64(),
         scan_duration.as_secs_f64() / total_duration.as_secs_f64() * 100.0);
-    println!("  - ID gen:    {:.2}s ({:.0}%)", 
+    println!("  - ID gen:        {:.2}s ({:.0}%)", 
         id_duration.as_secs_f64(),
         id_duration.as_secs_f64() / total_duration.as_secs_f64() * 100.0);
-    println!("  - DB open:   {:.2}s ({:.0}%)", 
+    println!("  - DB open:       {:.2}s ({:.0}%)", 
         db_duration.as_secs_f64(),
         db_duration.as_secs_f64() / total_duration.as_secs_f64() * 100.0);
-    println!("  - Insertion: {:.2}s ({:.0}%)", 
+    println!("  - Insertion:     {:.2}s ({:.0}%)", 
         insert_duration.as_secs_f64(),
         insert_duration.as_secs_f64() / total_duration.as_secs_f64() * 100.0);
+    println!("  - Children JSON: {:.2}s ({:.0}%)", 
+        children_duration.as_secs_f64(),
+        children_duration.as_secs_f64() / total_duration.as_secs_f64() * 100.0);
     
     if !options.skip_hash {
         println!();