fix: added chunks in contacts response

[edospadoni]

Summary

Implements chunked loading for phonebook contacts to prevent memory exhaustion and timeouts with large contact lists.

Changes

• Chunking: Process contacts in 8K chunks instead of loading all at once
• Resource limits: Added 256M memory limit, 400MB usage cap, 45s timeout protection
• Retry logic: Automatic retry (max 2) for failed chunk requests
• If-Modified-Since fix: Fallback to $_SERVER['HTTP_IF_MODIFIED_SINCE'] when apache_request_headers() fails
• Error handling: cURL error detection, JSON encoding validation, detailed logging
• Memory optimization: Free memory after each chunk, optimized contact data structure

Why

Large phonebooks (1000s of contacts) caused timeouts and memory exhaustion. Chunked processing with retry and resource monitoring resolves these issues.