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/pages/ServiceInfoPage.jsx

@@ -0,0 +1,133 @@
+import React, { useEffect, useState } from "react";
+import { apiFetch } from "../api";
+import { useAuth } from "../state";
+
+function formatUptime(seconds) {
+  const total = Math.max(0, Number(seconds || 0));
+  const d = Math.floor(total / 86400);
+  const h = Math.floor((total % 86400) / 3600);
+  const m = Math.floor((total % 3600) / 60);
+  const s = total % 60;
+  if (d > 0) return `${d}d ${h}h ${m}m`;
+  if (h > 0) return `${h}h ${m}m ${s}s`;
+  return `${m}m ${s}s`;
+}
+
+export function ServiceInfoPage() {
+  const { tokens, refresh } = useAuth();
+  const [info, setInfo] = useState(null);
+  const [message, setMessage] = useState("");
+  const [error, setError] = useState("");
+  const [busy, setBusy] = useState(false);
+
+  const load = async () => {
+    setError("");
+    const data = await apiFetch("/service/info", {}, tokens, refresh);
+    setInfo(data);
+  };
+
+  useEffect(() => {
+    load().catch((e) => setError(String(e.message || e)));
+  }, []);
+
+  const checkNow = async () => {
+    try {
+      setBusy(true);
+      setError("");
+      setMessage("");
+      const result = await apiFetch("/service/info/check", { method: "POST" }, tokens, refresh);
+      await load();
+      if (result.last_check_error) {
+        setMessage(`Version check finished with warning: ${result.last_check_error}`);
+      } else if (result.update_available) {
+        setMessage(`Update available: ${result.latest_version}`);
+      } else {
+        setMessage("Version check completed. No update detected.");
+      }
+    } catch (e) {
+      setError(String(e.message || e));
+    } finally {
+      setBusy(false);
+    }
+  };
+
+  if (!info) {
+    return <div className="card">Loading service information...</div>;
+  }
+
+  return (
+    <div>
+      <h2>Service Information</h2>
+      <p className="muted">Runtime details, installed version, and update check status for this NexaPG instance.</p>
+      {error && <div className="card error">{error}</div>}
+      {message && <div className="test-connection-result ok">{message}</div>}
+
+      <div className="grid three">
+        <div className="card">
+          <h3>Application</h3>
+          <div className="overview-kv">
+            <span>App Name</span>
+            <strong>{info.app_name}</strong>
+            <span>Environment</span>
+            <strong>{info.environment}</strong>
+            <span>API Prefix</span>
+            <strong>{info.api_prefix}</strong>
+          </div>
+        </div>
+        <div className="card">
+          <h3>Runtime</h3>
+          <div className="overview-kv">
+            <span>Host</span>
+            <strong>{info.hostname}</strong>
+            <span>Python</span>
+            <strong>{info.python_version}</strong>
+            <span>Uptime</span>
+            <strong>{formatUptime(info.uptime_seconds)}</strong>
+          </div>
+        </div>
+        <div className="card">
+          <h3>Version Status</h3>
+          <div className="overview-kv">
+            <span>Current NexaPG Version</span>
+            <strong>{info.app_version}</strong>
+            <span>Latest Known Version</span>
+            <strong>{info.latest_version || "-"}</strong>
+            <span>Update Status</span>
+            <strong className={info.update_available ? "lag-bad" : "pill primary"}>
+              {info.update_available ? "Update available" : "Up to date"}
+            </strong>
+            <span>Last Check</span>
+            <strong>{info.last_checked_at ? new Date(info.last_checked_at).toLocaleString() : "never"}</strong>
+          </div>
+          <div className="form-actions" style={{ marginTop: 12 }}>
+            <button type="button" className="secondary-btn" disabled={busy} onClick={checkNow}>
+              Check for Updates
+            </button>
+          </div>
+        </div>
+      </div>
+
+      <div className="card">
+        <h3>Release Source</h3>
+        <p className="muted">
+          Update checks run against the official NexaPG repository. This source is fixed in code and cannot be changed
+          via UI.
+        </p>
+        <div className="overview-kv">
+          <span>Source Repository</span>
+          <strong>{info.update_source}</strong>
+          <span>Latest Reference Type</span>
+          <strong>{info.latest_ref || "-"}</strong>
+        </div>
+      </div>
+
+      <div className="card">
+        <h3>Version Control Policy</h3>
+        <p className="muted">
+          Version and update-source settings are not editable in the app. Only code maintainers of the official NexaPG
+          repository can change that behavior.
+        </p>
+      </div>
+    </div>
+  );
+}