Get batch number from file contents instead of filename

src/main.rs

@@ -178,8 +178,12 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         batch.filename
     );
 
-    let batch_number = batch.filename.trim_end_matches(".csv");
-    let output_dir = base_output_dir.join(batch_number);
+    let batch_number = batch
+        .transactions
+        .first()
+        .map(|t| t.batch_number.clone())
+        .unwrap_or_else(|| "unknown".to_string());
+    let output_dir = base_output_dir.join(&batch_number);
 
     fs::create_dir_all(&output_dir)?;