fix: P0 stabilisation — score modifiers, logout cache clear, provider isolation, mutation error feedback

- scoreCalculator: apply dataQuality/confidence modifiers to finalScore (were computed but hardcoded to 0)
- authService: call queryClient.clear() on logout to prevent cross-session data leakage
- queryClient: extract to src/lib/queryClient.ts singleton so services can access it without circular imports
- matchSyncService: new service layer owns match-generation logic; MockupNeedProvider no longer imports other providers directly
- hooks (11 files): add onError + German toast feedback to every useMutation

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

src/hooks/useMarketSignals.ts

@@ -2,6 +2,7 @@ import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
 import { marketIntelligenceService } from '../services/marketIntelligenceService'
 import { reviewService } from '../services/reviewService'
 import type { MarketSignalFilters, SignalProcessingStatus } from '../domain/marketSignal'
+import { useToastStore } from '../stores/toastStore'
 
 const STALE_SIGNALS = 30_000
 
@@ -33,6 +34,9 @@ export function useUpdateSignalStatus() {
       queryClient.invalidateQueries({ queryKey: ['market-signals'] })
       queryClient.invalidateQueries({ queryKey: ['market-signal'] })
     },
+    onError: () => {
+      useToastStore.getState().showToast('Signal-Status konnte nicht aktualisiert werden.', 'error')
+    },
   })
 }
 
@@ -44,6 +48,9 @@ export function useConvertToFutureSignal() {
       queryClient.invalidateQueries({ queryKey: ['market-signals'] })
       queryClient.invalidateQueries({ queryKey: ['market-signal'] })
     },
+    onError: () => {
+      useToastStore.getState().showToast('Konvertierung zu Future Signal fehlgeschlagen.', 'error')
+    },
   })
 }
 
@@ -63,11 +70,17 @@ export function useLinkSignalToEntity() {
       queryClient.invalidateQueries({ queryKey: ['market-signals'] })
       queryClient.invalidateQueries({ queryKey: ['market-signal'] })
     },
+    onError: () => {
+      useToastStore.getState().showToast('Signal konnte nicht verknüpft werden.', 'error')
+    },
   })
 }
 
 export function useCreateReviewTask() {
   return useMutation({
     mutationFn: (signalId: string) => reviewService.createReviewTask(signalId),
+    onError: () => {
+      useToastStore.getState().showToast('Überprüfungsaufgabe konnte nicht erstellt werden.', 'error')
+    },
   })
 }