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/api/router.py

@@ -1,5 +1,5 @@
 from fastapi import APIRouter
-from app.api.routes import admin_settings, admin_users, alerts, auth, health, me, targets
+from app.api.routes import admin_settings, admin_users, alerts, auth, health, me, service_info, targets
 
 api_router = APIRouter()
 api_router.include_router(health.router, tags=["health"])
@@ -7,5 +7,6 @@ api_router.include_router(auth.router, prefix="/auth", tags=["auth"])
 api_router.include_router(me.router, tags=["auth"])
 api_router.include_router(targets.router, prefix="/targets", tags=["targets"])
 api_router.include_router(alerts.router, prefix="/alerts", tags=["alerts"])
+api_router.include_router(service_info.router, prefix="/service", tags=["service"])
 api_router.include_router(admin_users.router, prefix="/admin/users", tags=["admin"])
 api_router.include_router(admin_settings.router, prefix="/admin/settings", tags=["admin"])