fix: convert DECIMAL columns to numbers in fuel logs API (refs #49)

PostgreSQL DECIMAL columns return as strings from pg driver.
- Add Number() conversion for fuelUnits and costPerUnit in toEnhancedResponse()
- Add query invalidation for 'all' key to fix dynamic updates

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/features/fuel-logs/hooks/useFuelLogs.tsx

@@ -45,6 +45,7 @@ export const useFuelLogs = (vehicleId?: string) => {
     mutationFn: (data: CreateFuelLogRequest) => fuelLogsApi.create(data),
     onSuccess: (_res, variables) => {
       queryClient.invalidateQueries({ queryKey: ['fuelLogs', variables.vehicleId] });
+      queryClient.invalidateQueries({ queryKey: ['fuelLogs', 'all'] });
       queryClient.invalidateQueries({ queryKey: ['fuelLogsStats', variables.vehicleId] });
     },
   });