Add service information feature with version checks

This commit introduces a new "Service Information" section displaying runtime details, installed version, and update status for the NexaPG application. It includes backend API endpoints, database schema changes, and a corresponding frontend page that allows users to check for updates against the official repository. The `.env` example now includes an `APP_VERSION` variable, and related documentation has been updated.

backend/app/models/__init__.py

@@ -5,6 +5,7 @@ from app.models.models import (
     EmailNotificationSettings,
     Metric,
     QueryStat,
+    ServiceInfoSettings,
     Target,
     TargetOwner,
     User,
@@ -15,6 +16,7 @@ __all__ = [
     "Target",
     "Metric",
     "QueryStat",
+    "ServiceInfoSettings",
     "AuditLog",
     "AlertDefinition",
     "EmailNotificationSettings",