Address high memory usage when opening resource tab (fixes #5183) by implementing:

1. Concurrency control: Limit thumbnail generation to 3 concurrent operations using semaphore to prevent memory exhaustion when many images are requested simultaneously

2. S3 optimization: Skip server-side thumbnail generation for S3-stored images by default. S3 images now use presigned URLs directly, avoiding:
   - Downloading large images from S3 into server memory
   - Decoding and resizing images on the server
   - High memory consumption during batch requests

3. Memory management improvements:
   - Explicitly clear blob and decoded image from memory after use
   - Restructure thumbnail cache check to avoid unnecessary semaphore acquisition
   - Double-check pattern to prevent duplicate generation while waiting

This restores the original S3 behavior before commit e4f63453 while maintaining thumbnail support for local/database storage.

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Co-Authored-By: Claude <noreply@anthropic.com>