Remove configurable APP_VERSION and define it in code

The APP_VERSION variable is no longer configurable via the `.env` file or environment settings. Instead, the version is now hardcoded in `backend/app/core/config.py` as `NEXAPG_VERSION` and accessed through a property. This change simplifies version control and ensures consistency across deployments.

backend/app/core/config.py

@@ -2,12 +2,13 @@ from functools import lru_cache
 from pydantic import field_validator
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
+NEXAPG_VERSION = "0.1.1"
+
 
 class Settings(BaseSettings):
     model_config = SettingsConfigDict(env_file=".env", env_file_encoding="utf-8", extra="ignore")
 
     app_name: str = "NexaPG Monitor"
-    app_version: str = "0.1.0"
     environment: str = "dev"
     api_v1_prefix: str = "/api/v1"
     log_level: str = "INFO"
@@ -33,6 +34,10 @@ class Settings(BaseSettings):
     init_admin_email: str = "admin@example.com"
     init_admin_password: str = "ChangeMe123!"
 
+    @property
+    def app_version(self) -> str:
+        return NEXAPG_VERSION
+
     @property
     def database_url(self) -> str:
         return (