fix: add stock_calculation_mode ALTER migration for backward compatibility (#39)

Older production databases were missing the stock_calculation_mode column,
causing 500 errors on /export endpoint. Added migration to add column
with default value 'automatic'.

backend/src/db/client.ts

@@ -73,6 +73,8 @@ export async function runAlterMigrations(client: Client): Promise<{ success: boo
     `ALTER TABLE user_settings ADD COLUMN max_nagging_reminders integer NOT NULL DEFAULT 5`,
     // Added in v1.2.3 - dismiss missed doses without deducting stock
     `ALTER TABLE dose_tracking ADD COLUMN dismissed integer NOT NULL DEFAULT 0`,
+    // Added in v1.3.x - stock calculation mode (automatic/manual)
+    `ALTER TABLE user_settings ADD COLUMN stock_calculation_mode text NOT NULL DEFAULT 'automatic'`,
   ];
 
   for (const sql of alterMigrations) {