created the validation service and swagger set up

[yusuftomilola]

Description:

Summary

• Added Joi-based environment variable validation that fails fast on startup if any required config (DB, JWT, Cloudinary,
  Redis) is missing or invalid
• Added Swagger decorators (@ApiProperty, @ApiPropertyOptional) to payments and subscriptions DTOs and entities
• Created standalone Swagger config module with JWT bearer auth, named servers, and persistent authorization

closes #161
closes #162