Feat[BE]: refactor expense API and expense table match with new ERD

internal/database/migrations/20251117034511_create_expenses_table.up.sql

@@ -1,7 +1,7 @@
 CREATE TABLE expenses (
     id BIGSERIAL PRIMARY KEY,
     reference_number VARCHAR(50) UNIQUE NOT NULL,
-    supplier_id BIGINT NULL,
+    supplier_id BIGINT NOT NULL,
     category VARCHAR(50) NOT NULL CHECK (
         category IN ('BOP', 'NON-BOP')
     ),