perf(cache): stable cache keys and graceful shutdown fix (#95)

- Use JSON.stringify with replacer for stable key ordering
  This preserves JSON.stringify semantics (Date.toJSON, undefined omission,
  etc.) while ensuring {a:1, b:2} and {b:2, a:1} produce the same cache key

- Call unref() on cleanup interval so it doesn't prevent graceful shutdown
  Without this, the hourly interval keeps the Node.js event loop alive
  even when the server is trying to shut down

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>

Author: mbuckingham74

backend/services/cacheService.js

@@ -26,14 +26,39 @@ const CACHE_TTL = {
   CLIMATE_STATS: msToMinutes(TIMEOUTS.CACHE.CLIMATE_TTL, 43200),
 };
 
+/**
+ * Stable JSON stringify with sorted keys
+ * Uses JSON.stringify with a replacer to ensure consistent key ordering.
+ * Handles Date (via toJSON), undefined (omitted), and other JSON.stringify semantics.
+ * @param {any} obj - Object to stringify
+ * @returns {string} Stable JSON string
+ */
+function stableStringify(obj) {
+  // Use JSON.stringify with a replacer that sorts object keys
+  // This preserves JSON.stringify semantics (Date.toJSON, undefined handling, etc.)
+  return JSON.stringify(obj, (key, value) => {
+    // For objects (not arrays, not null), return a new object with sorted keys
+    if (value && typeof value === 'object' && !Array.isArray(value)) {
+      return Object.keys(value)
+        .sort()
+        .reduce((sorted, k) => {
+          sorted[k] = value[k];
+          return sorted;
+        }, {});
+    }
+    return value;
+  });
+}
+
 /**
  * Generate cache key from request parameters
+ * Uses stable stringify to ensure consistent keys regardless of object key order
  * @param {string} apiSource - API source identifier (e.g., 'visualcrossing')
  * @param {object} params - Request parameters
  * @returns {string} MD5 hash cache key
  */
 function generateCacheKey(apiSource, params) {
-  const paramsString = JSON.stringify(params);
+  const paramsString = stableStringify(params);
   return crypto.createHash('md5').update(`${apiSource}:${paramsString}`).digest('hex');
 }
 
@@ -212,6 +237,10 @@ if (process.env.NODE_ENV !== 'test') {
     },
     60 * 60 * 1000
   ); // 1 hour
+
+  // Allow process to exit even if this interval is still running
+  // Without this, the interval keeps the event loop alive during graceful shutdown
+  cacheCleanupInterval.unref();
 }
 
 module.exports = {