global error handler

[samad13]

1. Created custom error classes in src/errors/AppError.ts :
   • Base class AppError that extends Error , has statusCode and isOperational flag
   • Helper classes for common HTTP errors: NotFoundError , BadRequestError , UnauthorizedError , ForbiddenError , ConflictError
2. Enhanced the global error handling middleware in src/middleware/errorHandler.ts :
   • Added handlers for Mongoose-specific errors (CastError, Duplicate Fields, ValidationError)
   • Added separate sendErrorDev and sendErrorProd functions that format errors appropriately for each environment
   • Updated to use our env.ts config for NODE_ENV check
3. Re-created src/config/env.ts (oops, it got lost earlier!) that validates env variables with Zod
4. Verified everything works by running tests and lint, all of which pass!
   The implementation follows the strict layered architecture, is production-ready with proper error handling and strong TypeScript types!
   Closes Backend: Set up global error handling middleware and custom Express error classes #5

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