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/useMatches.ts

@@ -1,6 +1,7 @@
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
 import { matchService } from '../services/matchService'
 import { STALE_MATCHES } from '../lib/constants'
+import { useToastStore } from '../stores/toastStore'
 
 export function useMatches() {
   return useQuery({
@@ -38,6 +39,9 @@ export function useApproveMatch() {
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ['matches'] })
     },
+    onError: () => {
+      useToastStore.getState().showToast('Match konnte nicht genehmigt werden.', 'error')
+    },
   })
 }