Add cache clearing of moderation requests

This removes the memory leak of not removing the moderation api cache

src/moderation.ts

@@ -1,10 +1,13 @@
-import { Collection, InteractionResponse, Message } from "discord.js";
+import { Collection, InteractionResponse, Message, SnowflakeUtil } from "discord.js";
+
 import { openai } from "./index";
 import { formatRequestOrResponse } from "./toOpenAIMessages";
+import config from "./config";
 
 export default class Moderation {
   /** Represents cache of messages that have been checked aganist OpenAI moderation API. */
   private static cache = new Collection<string, boolean>();
+  private static worker: NodeJS.Timeout | null = null;
 
   public static async checkMessage(message: Message | InteractionResponse): Promise<boolean> {
     if (this.cache.has(message.id)) {
@@ -43,4 +46,29 @@ export default class Moderation {
       return false;
     }
   }
+
+  public static removeExpiredCacheEntries() {
+    const now = Date.now();
+    for (const i of this.cache.keys()) {
+      if (now - SnowflakeUtil.timestampFrom(i) >= config.limits.time * 2) {
+        this.cache.delete(i);
       }
+    }
+  }
+  public static startCacheClearWorker() {
+    if (this.worker) return;
+    this.worker = setInterval(() => {
+      this.removeExpiredCacheEntries();
+    },
+    1000 * 60 * 15 /* 15 minutes */,
+    );
+  }
+
+  public static stopCacheClearWorker() {
+    if (!this.worker) return;
+    clearInterval(this.worker);
+    this.worker = null;
+  }
+}
+
+Moderation.startCacheClearWorker();