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>

backend/src/features/fuel-logs/domain/fuel-logs.service.ts

@@ -282,8 +282,8 @@ export class FuelLogsService {
       tripDistance: row.trip_distance ?? undefined,
       fuelType: row.fuel_type as FuelType,
       fuelGrade: row.fuel_grade ?? undefined,
-      fuelUnits: row.fuel_units,
-      costPerUnit: row.cost_per_unit,
+      fuelUnits: Number(row.fuel_units),
+      costPerUnit: Number(row.cost_per_unit),
       totalCost: Number(row.total_cost),
       locationData: row.location_data ?? undefined,
       notes: row.notes ?? undefined,