feat: spread feed fetch scheduling deterministically over the hour

Previously, all RSS feeds were fetched at the top of each hour (minute 0),
which could cause load spikes. This change spreads feed fetches evenly
throughout the hour using a deterministic hash of the feed ID.

Each feed is assigned a target minute (0-59) based on its ID hash, ensuring
consistent scheduling across restarts while distributing the load evenly.

apps/workers/workers/feedWorker.ts

@@ -14,6 +14,21 @@ import logger from "@karakeep/shared/logger";
 import { DequeuedJob, getQueueClient } from "@karakeep/shared/queueing";
 import { BookmarkTypes } from "@karakeep/shared/types/bookmarks";
 
+/**
+ * Deterministically maps a feed ID to a minute offset within the hour (0-59).
+ * This ensures feeds are spread evenly across the hour based on their ID.
+ */
+function getFeedMinuteOffset(feedId: string): number {
+  // Simple hash function: sum character codes
+  let hash = 0;
+  for (let i = 0; i < feedId.length; i++) {
+    hash = (hash << 5) - hash + feedId.charCodeAt(i);
+    hash = hash & hash; // Convert to 32-bit integer
+  }
+  // Return a minute offset between 0 and 59
+  return Math.abs(hash) % 60;
+}
+
 export const FeedRefreshingWorker = cron.schedule(
   "0 * * * *",
   () => {
@@ -30,9 +45,24 @@ export const FeedRefreshingWorker = cron.schedule(
         const currentHour = new Date();
         currentHour.setMinutes(0, 0, 0);
         const hourlyWindow = currentHour.toISOString();
+        const now = new Date();
+        const currentMinute = now.getMinutes();
 
         for (const feed of feeds) {
           const idempotencyKey = `${feed.id}-${hourlyWindow}`;
+          const targetMinute = getFeedMinuteOffset(feed.id);
+
+          // Calculate delay: if target minute has passed, schedule for next hour
+          let delayMinutes = targetMinute - currentMinute;
+          if (delayMinutes < 0) {
+            delayMinutes += 60;
+          }
+          const delayMs = delayMinutes * 60 * 1000;
+
+          logger.debug(
+            `[feed] Scheduling feed ${feed.id} at minute ${targetMinute} (delay: ${delayMinutes} minutes)`,
+          );
+
           FeedQueue.enqueue(
             {
               feedId: feed.id,
@@ -40,6 +70,7 @@ export const FeedRefreshingWorker = cron.schedule(
             {
               idempotencyKey,
               groupId: feed.userId,
+              delayMs,
             },
           );
         }