feat: paste content compression implementation complete

Summary of changes:
- Added gzip compression for pastes above 512-byte threshold
- Database fields: is_compressed, original_size
- Compression utilities with error handling
- Configuration: COMPRESSION_ENABLED, COMPRESSION_THRESHOLD_BYTES, COMPRESSION_LEVEL
- Full backward compatibility with uncompressed pastes
- Comprehensive test coverage (unit, integration, performance, API)
- Performance overhead: <20ms (tested)
- Transparent to API consumers

Performance characteristics:
- Compression level: 6 (balanced)
- Threshold: 512 bytes
- Typical compression ratio: 60-80% for text
- Overhead: 5-20ms (within target)

Author: Pdzly