feat(api): add /api/radio stub returning the seed track only

M6 stub. Validates seed_track param (400 on missing/blank/bad UUID),
looks up the track (404 on miss), returns RadioResponse with a single
TrackRef. M4 will replace the body with similarity-driven candidate
pool + scoring; the request/response shape is final.

internal/api/api.go

@@ -32,6 +32,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger) {
 			authed.Get("/tracks/{id}", h.handleGetTrack)
 			authed.Get("/tracks/{id}/stream", h.handleGetStream)
 			authed.Get("/search", h.handleSearch)
+			authed.Get("/radio", h.handleRadio)
 		})
 	})
 }