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/atemschutz/AtemschutzDashboardCard.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 { atemschutzApi } from '../../services/atemschutz';
 import type { AtemschutzStats } from '../../types/atemschutz.types';
 
@@ -20,31 +21,12 @@ interface AtemschutzDashboardCardProps {
 const AtemschutzDashboardCard: React.FC<AtemschutzDashboardCardProps> = ({
   hideWhenEmpty = false,
 }) => {
-  const [stats, setStats] = useState<AtemschutzStats | null>(null);
-  const [loading, setLoading] = useState(true);
-  const [error, setError] = useState<string | null>(null);
+  const { data: stats, isLoading, isError } = useQuery<AtemschutzStats>({
+    queryKey: ['atemschutz-stats'],
+    queryFn: () => atemschutzApi.getStats(),
+  });
 
-  useEffect(() => {
-    let mounted = true;
-    const fetchStats = async () => {
-      try {
-        setLoading(true);
-        setError(null);
-        const data = await atemschutzApi.getStats();
-        if (mounted) setStats(data);
-      } catch {
-        if (mounted) setError('Atemschutzstatus konnte nicht geladen werden.');
-      } finally {
-        if (mounted) setLoading(false);
-      }
-    };
-    fetchStats();
-    return () => {
-      mounted = false;
-    };
-  }, []);
-
-  if (loading) {
+  if (isLoading) {
     return (
       <Card>
         <CardContent sx={{ display: 'flex', alignItems: 'center', gap: 1, py: 2 }}>
@@ -57,12 +39,12 @@ const AtemschutzDashboardCard: React.FC<AtemschutzDashboardCardProps> = ({
     );
   }
 
-  if (error) {
+  if (isError) {
     return (
       <Card>
         <CardContent>
           <Typography variant="body2" color="error">
-            {error}
+            Atemschutzstatus konnte nicht geladen werden.
           </Typography>
         </CardContent>
       </Card>
@@ -71,7 +53,6 @@ const AtemschutzDashboardCard: React.FC<AtemschutzDashboardCardProps> = ({
 
   if (!stats) return null;
 
-  // Determine if there are any concerns
   const hasConcerns =
     stats.untersuchungAbgelaufen > 0 ||
     stats.leistungstestAbgelaufen > 0 ||
@@ -80,7 +61,6 @@ const AtemschutzDashboardCard: React.FC<AtemschutzDashboardCardProps> = ({
 
   const allGood = stats.einsatzbereit === stats.total && !hasConcerns;
 
-  // If hideWhenEmpty and everything is fine, render nothing
   if (hideWhenEmpty && allGood) return null;
 
   return (