fix: dashboard layout, widget caching, and backend stability

Layout:
- Remove Container maxWidth cap so widgets scale fluidly on wide screens
- Fix ActivityFeed Card missing height:100% and overflow:hidden that caused
  the timeline connector pseudo-element to bleed outside the card boundary

Performance (frontend):
- Migrate VehicleDashboardCard, EquipmentDashboardCard, AtemschutzDashboardCard,
  UpcomingEventsWidget, and PersonalWarningsBanner from useEffect+useState to
  TanStack Query — cached for 5 min, so navigating back to the dashboard no
  longer re-fires all 9 API requests
- Add gcTime:10min and refetchOnWindowFocus:false to QueryClient defaults to
  prevent spurious refetches on tab-switch

Backend stability:
- Raise default RATE_LIMIT_MAX from 100 to 300 req/15min — the previous limit
  was easily exceeded by a single active user during normal dashboard navigation
- Increase DB connectionTimeoutMillis from 2s to 5s to handle burst-load
  scenarios where multiple requests compete for pool slots simultaneously

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/vehicles/VehicleDashboardCard.tsx

@@ -1,4 +1,4 @@
-import React, { useEffect, useState } from 'react';
+import React from 'react';
 import {
   Alert,
   AlertTitle,
@@ -10,6 +10,7 @@ import {
   Typography,
 } from '@mui/material';
 import { Link as RouterLink } from 'react-router-dom';
+import { useQuery } from '@tanstack/react-query';
 import { vehiclesApi } from '../../services/vehicles';
 import { equipmentApi } from '../../services/equipment';
 import type { VehicleStats, InspectionAlert } from '../../types/vehicle.types';
@@ -22,39 +23,22 @@ interface VehicleDashboardCardProps {
 const VehicleDashboardCard: React.FC<VehicleDashboardCardProps> = ({
   hideWhenEmpty = false,
 }) => {
-  const [stats, setStats] = useState<VehicleStats | null>(null);
-  const [alerts, setAlerts] = useState<InspectionAlert[]>([]);
-  const [equipmentWarnings, setEquipmentWarnings] = useState<VehicleEquipmentWarning[]>([]);
-  const [loading, setLoading] = useState(true);
-  const [error, setError] = useState<string | null>(null);
+  const { data: stats, isLoading: statsLoading, isError: statsError } = useQuery<VehicleStats>({
+    queryKey: ['vehicle-stats'],
+    queryFn: () => vehiclesApi.getStats(),
+  });
 
-  useEffect(() => {
-    let mounted = true;
-    const fetchData = async () => {
-      try {
-        setLoading(true);
-        setError(null);
-        const [statsData, alertsData, warningsData] = await Promise.all([
-          vehiclesApi.getStats(),
-          vehiclesApi.getAlerts(30),
-          equipmentApi.getVehicleWarnings(),
-        ]);
-        if (mounted) {
-          setStats(statsData);
-          setAlerts(alertsData);
-          setEquipmentWarnings(warningsData);
-        }
-      } catch {
-        if (mounted) setError('Fahrzeugstatus konnte nicht geladen werden.');
-      } finally {
-        if (mounted) setLoading(false);
-      }
-    };
-    fetchData();
-    return () => {
-      mounted = false;
-    };
-  }, []);
+  const { data: alerts = [], isLoading: alertsLoading } = useQuery<InspectionAlert[]>({
+    queryKey: ['vehicle-alerts'],
+    queryFn: () => vehiclesApi.getAlerts(30),
+  });
+
+  const { data: equipmentWarnings = [], isLoading: warningsLoading } = useQuery<VehicleEquipmentWarning[]>({
+    queryKey: ['vehicle-equipment-warnings'],
+    queryFn: () => equipmentApi.getVehicleWarnings(),
+  });
+
+  const loading = statsLoading || alertsLoading || warningsLoading;
 
   if (loading) {
     return (
@@ -69,12 +53,12 @@ const VehicleDashboardCard: React.FC<VehicleDashboardCardProps> = ({
     );
   }
 
-  if (error) {
+  if (statsError) {
     return (
       <Card>
         <CardContent>
           <Typography variant="body2" color="error">
-            {error}
+            Fahrzeugstatus konnte nicht geladen werden.
           </Typography>
         </CardContent>
       </Card>
@@ -94,7 +78,6 @@ const VehicleDashboardCard: React.FC<VehicleDashboardCardProps> = ({
 
   const allGood = stats.einsatzbereit === stats.total && !hasConcerns;
 
-  // If hideWhenEmpty and everything is fine, render nothing
   if (hideWhenEmpty && allGood) return null;
 
   return (