fix(billing): compute plan expiry with Jalali months

Server/plugin/support charges used Gregorian Date#setMonth, so 3 months from 1405/06/01 became 1405/09/02. Now add N Jalali months minus one day (1405/08/30).

package.json

@@ -57,6 +57,7 @@
     "bcrypt": "5.1.1",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.14.2",
+    "jalaali-js": "^2.0.1",
     "mysql2": "^3.23.1",
     "passport": "^0.7.0",
     "passport-jwt": "^4.0.1",
@@ -71,6 +72,7 @@
     "@nestjs/testing": "^11.0.1",
     "@types/bcrypt": "5.0.2",
     "@types/express": "^5.0.0",
+    "@types/jalaali-js": "^1.2.0",
     "@types/jest": "^30.0.0",
     "@types/multer": "^2.2.0",
     "@types/node": "^22.0.0",