Shelf to Relic Migration

Relic replaces shelf, shelf_router, and shelf_web_socket with a single package. Core concepts (handlers, middleware, requests, responses) carry over with improved type safety.

1. Dependencies

Remove Shelf packages, add Relic:

# Before (Shelf)
dependencies:
  shelf: <version>
  shelf_router: <version>
  shelf_web_socket: <version>

# After (Relic)
dependencies:
  relic: <latest_version>