fix: add unique constraint to split and dividends

to prevent duplicate records

database/migrations/2021_02_25_041246_create_splits_table.php

@@ -21,6 +21,8 @@ class CreateSplitsTable extends Migration
             $table->string('symbol', 25);
             $table->float('split_amount', 12, 4);
             $table->timestamps();
+
+            $table->unique(['date', 'symbol']);
         });
     }