perf: cache slow-changing data (series, movies, tags) with 60s TTL

- Add MemoryCache utility with get/set/invalidate/clear
- Cache Sonarr series, Sonarr tags, Radarr movies, Radarr tags
- 60-second TTL - first request fetches, subsequent requests served from cache
- Queue, history, and torrent data remain uncached (changes frequently)
- On cache hit, these 4 heavy API calls resolve instantly

server/utils/cache.js

@@ -0,0 +1,36 @@
+const { logToFile } = require('./logger');
+
+class MemoryCache {
+  constructor() {
+    this.store = new Map();
+  }
+
+  get(key) {
+    const entry = this.store.get(key);
+    if (!entry) return null;
+    if (Date.now() > entry.expiresAt) {
+      this.store.delete(key);
+      return null;
+    }
+    return entry.value;
+  }
+
+  set(key, value, ttlMs) {
+    this.store.set(key, {
+      value,
+      expiresAt: Date.now() + ttlMs
+    });
+  }
+
+  invalidate(key) {
+    this.store.delete(key);
+  }
+
+  clear() {
+    this.store.clear();
+  }
+}
+
+const cache = new MemoryCache();
+
+module.exports = cache;