Compare users across ranked shelves and watchlists

[ALeonard9]

Summary

• add viewer-safe cross-profile comparisons for movies, TV, books, and games
• respect public, friends-only, and private visibility independently for ranked shelves and watchlists
• return alignment, biggest gaps, closest agreement, and the other user’s top unseen picks
• let recommendations be saved to a watchlist or rank list without launching a duel
• record who inspired saved recommendations and expose that provenance on tracker responses

Verification

• full API suite: 791 passed
• changed-test pre-push suite passed
• migration applied against the local development database

Closes #281