feat(feed): genre affinity scoring for the for-you feed

[dylanjeffers]

Item 3 of the for-you feed improvement queue (follows #949 content-type weighting and #950 repeat suppression).

What

Adds a genre_affinity multiplier to final_score, built from the genre mix of the viewer's recent listening:

• New `my_genre_affinity` CTE: joins the viewer's most recent plays (bounded to the last 1000, same pattern as the other play sub-selects) to `tracks.genre` and computes each genre's share of plays.
• Multiplier: `0.85 + 0.45 * min(genre_share / 0.30, 1)` — a genre at >=30% of recent listening gets the full 1.30x, genres the viewer never plays get 0.85x.
• Neutral 1.00x for playlists (no single genre), genre-less tracks, and cold-start users with no play history (no penalty when we know nothing).

Tests

• `TestV1FeedForYou_GenreAffinityBoostsFavoredGenre`: an all-Electronic listener sees the Electronic sibling outrank the naturally-stronger Rock sibling, while the Rock track stays in the feed.
• `TestV1FeedForYou_GenreAffinityNeutralOnColdStart`: no play history → natural ranking unchanged.

`go build ./...` and the full `go test ./...` suite pass locally.

🤖 Opened by the automated feed-algorithm-improvement-loop.