Refactor phone number validation and enhance test coverage

Improved phone number validation logic with stricter rules and better error messages in `UserBase`. Updated access token expiration to 1 day in config. Added extensive tests for phone number validation, including valid and invalid cases across different formats.

backend/app/core/config.py

@@ -41,7 +41,7 @@ class Settings(BaseSettings):
     # JWT configuration
     SECRET_KEY: str = "your_secret_key_here"
     ALGORITHM: str = "HS256"
-    ACCESS_TOKEN_EXPIRE_MINUTES: int = 30
+    ACCESS_TOKEN_EXPIRE_MINUTES: int = 1440 # 1 day
 
     # CORS configuration
     BACKEND_CORS_ORIGINS: List[str] = ["http://localhost:3000"]