feat: timer intervallo misura su ricetta + auto-logout per inattività

- Recipe: nuovo campo measurement_interval_minutes (migration 003) con UI nel recipe editor
- Auto-logout: impostazione di sistema configurabile da admin/settings, timer inattività JS
  con warning modale 60s prima del logout, tracking eventi utente throttled
- Navbar: aggiunto link "Impostazioni" per admin (desktop + mobile)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/frontend/flask_app/blueprints/admin.py

@@ -43,6 +43,28 @@ def user_list():
     return render_template("admin/users.html", users=users)
 
 
+@admin_bp.route("/settings")
+@login_required
+@admin_required
+def settings_page():
+    """System settings page."""
+    resp = api_client.get("/api/settings")
+    settings = resp if not isinstance(resp, dict) or not resp.get("error") else {}
+    return render_template("admin/settings.html", settings=settings)
+
+
+@admin_bp.route("/api/settings", methods=["PUT"])
+@login_required
+@admin_required
+def api_update_settings():
+    """Proxy: Update system settings."""
+    data = request.get_json(silent=True) or {}
+    resp = api_client.put("/api/settings", data=data)
+    if isinstance(resp, dict) and resp.get("error"):
+        return jsonify(resp), resp.get("status_code", 500)
+    return jsonify(resp), 200
+
+
 @admin_bp.route("/stations")
 @login_required
 @admin_required