FIX[BE}: really fixed duplicate SO number

internal/database/migrations/20251210044651_create_so_number_sequence.up.sql

@@ -0,0 +1,12 @@
+-- Create sequence for sales order numbers
+CREATE SEQUENCE so_number_seq START WITH 1 INCREMENT BY 1;
+
+CREATE OR REPLACE FUNCTION generate_so_number()
+RETURNS VARCHAR AS $$
+DECLARE
+    next_val INTEGER;
+BEGIN
+    next_val := nextval('so_number_seq');
+    RETURN 'SO-' || LPAD(next_val::TEXT, 5, '0');
+END;
+$$ LANGUAGE plpgsql;