Add zip bomb protection to ZipConverter

[mvanhorn]

Problem

ZipConverter reads each file in a ZIP archive by calling zipObj.read(name) without checking the decompressed size first. A malicious zip bomb (a small ZIP that decompresses to gigabytes) can crash the process with an out-of-memory error.

Reported in #1514.

Fix

Added three pre-read safety checks in ZipConverter.convert():

1. Per-file size limit (100 MB) - checks ZipInfo.file_size before reading
2. Decompression ratio limit (100:1) - compares file_size / compress_size to detect suspiciously high compression ratios typical of zip bombs
3. Cumulative size limit (500 MB) - tracks total decompressed bytes across all files in the archive

Files exceeding any limit are skipped with a logging.warning() message rather than raising an exception, so the rest of the archive is still processed (graceful degradation).

Testing

Added test_zip_security.py with four tests:

• Normal ZIP converts successfully (no regression)
• Oversized individual file is skipped
• High decompression ratio is detected and skipped
• Cumulative size limit stops extraction

All limits are defined as module-level constants (MAX_DECOMPRESSED_FILE_SIZE, MAX_DECOMPRESSION_RATIO, MAX_TOTAL_DECOMPRESSED_SIZE) for easy adjustment.

This contribution was developed with AI assistance (Claude Code).