feat(compression): API response compression & payload optimization (#129)

[dagangtj]

Summary

Reduce payload size and improve response times with gzip compression and payload optimization.

Closes #129

Changes

• Gzip middleware: packages/backend/app/middleware/compression.py

  • Auto-compresses JSON responses >500 bytes when client accepts gzip
  • Configurable minimum size and compression level
  • Only compresses when result is actually smaller
  • Adds proper Content-Encoding, Content-Length, Vary headers

• Payload optimization:

  • @slim_response decorator for field selection/exclusion
  • Query param ?fields=id,name for client-side field selection
  • add_pagination_headers() utility for X-Page/X-Total headers

• Tests: 12 test cases covering compression, filtering, pagination

Acceptance Criteria

• Production ready implementation
• Includes tests (12 cases)
• Follows existing code patterns