chore: code cleanup - remove duplicates and add caching

- Delete duplicate getCacheStats() function in cache.service.js
- Fix date calculation bug in lotw.service.js (was Date.now()-Date.now())
- Extract duplicate helper functions (yieldToEventLoop, getQSOKey) to sync-helpers.js
- Cache award definitions in memory to avoid repeated file I/O
- Delete unused parseDCLJSONResponse() function
- Remove unused imports (getPerformanceSummary, resetPerformanceMetrics)
- Auto-discover award JSON files instead of hardcoded list

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

src/backend/services/cache.service.js

@@ -86,32 +86,6 @@ export function clearAllCache() {
   return size;
 }
 
-/**
- * Get cache statistics (for monitoring/debugging)
- * @returns {object} Cache stats
- */
-export function getCacheStats() {
-  const now = Date.now();
-  let expired = 0;
-  let valid = 0;
-
-  for (const [, value] of awardCache) {
-    const age = now - value.timestamp;
-    if (age > CACHE_TTL) {
-      expired++;
-    } else {
-      valid++;
-    }
-  }
-
-  return {
-    total: awardCache.size,
-    valid,
-    expired,
-    ttl: CACHE_TTL
-  };
-}
-
 /**
  * Clean up expired cache entries (maintenance function)
  * Can be called periodically to free memory