feat: replace duplicate feed with copy feed functionality

- Add copy_feed SQL function that allows copying feeds from any user
- Support shallow copy (share follow lists) and deep copy (create independent copies)
- Feed generators are always shallow-copied (reference same definitions)
- Deep copy creates new follow lists with "(copy)" or "(from username)" suffix
- Update UI with radio buttons for copy mode selection
- Add preview showing what will be copied
- Update integration tests for shallow and deep copy modes

Closes #135