fix(scan): prevent exponential slowdown from event loop blocking

- Reduce default CONCURRENT_API_CALLS from 10 to 5 (Sharp AVIF is CPU-intensive)
- Create fresh p-limit instance per batch instead of module singleton
- Add garbage collection hint between batches
- Fix skippedCount tracking (was never incremented)
- Increase batch delay from 100ms to 500ms for event loop drainage

services/cache-manager.ts

@@ -25,14 +25,15 @@ const USERNAME = process.env.API_USERNAME;
 const PASSWORD = process.env.API_PASSWORD;
 const MIN_ACTIVITY_ID_SCAN = parseInt(process.env.MIN_ACTIVITY_ID_SCAN || '0', 10);
 const MAX_ACTIVITY_ID_SCAN = parseInt(process.env.MAX_ACTIVITY_ID_SCAN || '9999', 10);
-const CONCURRENT_API_CALLS = parseInt(process.env.CONCURRENT_API_CALLS || '10', 10);
+// Default 5 concurrent calls (was 10) - Sharp AVIF processing is CPU-intensive and can block event loop
+const CONCURRENT_API_CALLS = parseInt(process.env.CONCURRENT_API_CALLS || '5', 10);
 const CLUB_UPDATE_INTERVAL_MINS = parseInt(process.env.CLUB_UPDATE_INTERVAL_MINS || '60', 10);
 const STAFF_UPDATE_INTERVAL_MINS = parseInt(process.env.STAFF_UPDATE_INTERVAL_MINS || '60', 10);
 const FIXED_STAFF_ACTIVITY_ID = process.env.FIXED_STAFF_ACTIVITY_ID;
 const S3_IMAGE_PREFIX = (process.env.S3_PUBLIC_URL_PREFIX || 'files').replace(/\/$/, '');
 
-// Limit concurrent API calls
+// Module-level counter for skipped activities (reset at start of each scan)
-const limit = pLimit(CONCURRENT_API_CALLS);
+let skippedCount = 0;
 
 /**
  * Process and cache a single activity
@@ -105,8 +106,9 @@ async function processSingleActivity(activityId: string): Promise<void> {
     
     logger.debug(`Initializing cache for activity ID: ${activityId}`);
     await processAndCacheActivity(activityId);
+  } else {
+    skippedCount++;
   }
-  // else: skip (already cached)
 }
 
 /**
@@ -121,11 +123,13 @@ export async function initializeClubCache(): Promise<void> {
   let processedCount = 0;
   let successCount = 0;
   let errorCount = 0;
-  let skippedCount = 0;
+  skippedCount = 0; // Reset for this run
   
   for (let batchStart = MIN_ACTIVITY_ID_SCAN; batchStart <= MAX_ACTIVITY_ID_SCAN; batchStart += BATCH_SIZE) {
     const batchEnd = Math.min(batchStart + BATCH_SIZE - 1, MAX_ACTIVITY_ID_SCAN);
     const batchPromises: Promise<void>[] = [];
+    // Create fresh p-limit instance per batch to prevent internal queue accumulation
+    const limit = pLimit(CONCURRENT_API_CALLS);
     
     logger.info(`Processing batch ${Math.floor(processedCount / BATCH_SIZE) + 1}/${Math.ceil(totalIds / BATCH_SIZE)} (IDs ${batchStart}-${batchEnd})`);
     
@@ -158,8 +162,13 @@ export async function initializeClubCache(): Promise<void> {
     await Promise.allSettled(batchPromises);
     batchPromises.length = 0;
     
+    // Garbage collection hint and longer delay between batches to allow event loop to drain
+    if (global.gc) {
+      global.gc(false);
+    }
+    
     if (batchEnd < MAX_ACTIVITY_ID_SCAN) {
-      await new Promise(resolve => setTimeout(resolve, 100));
+      await new Promise(resolve => setTimeout(resolve, 500));
     }
   }
   
@@ -174,6 +183,7 @@ export async function updateStaleClubs(): Promise<void> {
   logger.info('Starting stale club check...');
   const now = Date.now();
   const updateIntervalMs = CLUB_UPDATE_INTERVAL_MINS * 60 * 1000;
+  const limit = pLimit(CONCURRENT_API_CALLS);
   const promises: Promise<void>[] = [];
   const activityKeys = await getAllActivityKeys();