feat: add API response compression and payload optimization

[gavin913427-hash]

Changes

Implement response compression and payload optimization.

Features

• Flask-Compress for automatic gzip compression (JSON, HTML, CSS, JS, XML)
• Configurable minimum size threshold (500 bytes)
• Response size tracking via Prometheus histogram
• Pagination helper utility for consistent API responses
• Updated expenses route to use pagination helper

Files Modified

• packages/backend/requirements.txt - Added flask-compress
• packages/backend/app/init.py - Configured compression
• packages/backend/app/observability.py - Added response size metrics
• packages/backend/app/routes/expenses.py - Uses pagination helper
• packages/backend/app/utils/init.py - NEW: Utils package
• packages/backend/app/utils/pagination.py - NEW: Pagination helper
• packages/backend/tests/test_compression.py - NEW: 7 tests
• packages/backend/app/services/README_COMPRESSION.md - NEW: Documentation

Fixes #129