feat(server/playlists): manual For-You refresh endpoint

POST /api/playlists/system/for-you/refresh re-runs the system
playlist build synchronously for the calling user, then returns
the freshly-built For-You playlist's id, track count, and the
track IDs in playlist position order.

The frontend tile play button (next task) calls this endpoint
on click and enqueues the returned track_ids directly — one
roundtrip, no follow-up "list tracks" call needed for playback
to start. Same authenticated-user-only posture as the Discover
refresh from D-T3.

Returns playlist_id=null and track_ids=[] when the build
succeeded but the user's library yielded no eligible candidates
(degenerate empty-library). Returns 500 on actual build failure.

Tests cover 200 with shape (track_count matches len(track_ids))
and 401 without auth.

internal/api/api.go

@@ -137,6 +137,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			authed.Put("/playlists/{id}/tracks", h.handleReorderPlaylist)
 			authed.Get("/playlists/{id}/cover", h.handleGetPlaylistCover)
 			authed.Post("/playlists/system/discover/refresh", h.handleDiscoverRefresh)
+			authed.Post("/playlists/system/for-you/refresh", h.handleForYouRefresh)
 		})
 	})
 }