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.

frontend/src/App.jsx

@@ -8,6 +8,7 @@ import { TargetDetailPage } from "./pages/TargetDetailPage";
 import { QueryInsightsPage } from "./pages/QueryInsightsPage";
 import { AlertsPage } from "./pages/AlertsPage";
 import { AdminUsersPage } from "./pages/AdminUsersPage";
+import { ServiceInfoPage } from "./pages/ServiceInfoPage";
 
 function Protected({ children }) {
   const { tokens } = useAuth();
@@ -61,6 +62,14 @@ function Layout({ children }) {
             </span>
             <span className="nav-label">Alerts</span>
           </NavLink>
+          <NavLink to="/service-info" className={navClass}>
+            <span className="nav-icon" aria-hidden="true">
+              <svg viewBox="0 0 24 24">
+                <path d="M12 22a10 10 0 1 0 0-20 10 10 0 0 0 0 20zm0-11v6m0-10h.01" />
+              </svg>
+            </span>
+            <span className="nav-label">Service Information</span>
+          </NavLink>
           {me?.role === "admin" && (
             <>
               <div className="sidebar-nav-spacer" aria-hidden="true" />
@@ -150,6 +159,7 @@ export function App() {
                 <Route path="/targets/:id" element={<TargetDetailPage />} />
                 <Route path="/query-insights" element={<QueryInsightsPage />} />
                 <Route path="/alerts" element={<AlertsPage />} />
+                <Route path="/service-info" element={<ServiceInfoPage />} />
                 <Route path="/admin/users" element={<AdminUsersPage />} />
               </Routes>
             </Layout>