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/radio.go

@@ -0,0 +1,60 @@
+package api
+
+import (
+	"errors"
+	"net/http"
+	"strings"
+
+	"github.com/jackc/pgx/v5"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// RadioResponse is the body of GET /api/radio.
+type RadioResponse struct {
+	Tracks []TrackRef `json:"tracks"`
+}
+
+// handleRadio implements GET /api/radio?seed_track=<uuid>.
+//
+// M6 stub: returns a queue containing only the seed track. The full M4
+// implementation (similarity-based candidate pool + scoring) replaces the
+// body without changing the request/response shape.
+func (h *handlers) handleRadio(w http.ResponseWriter, r *http.Request) {
+	raw := strings.TrimSpace(r.URL.Query().Get("seed_track"))
+	if raw == "" {
+		writeErr(w, http.StatusBadRequest, "bad_request", "seed_track is required")
+		return
+	}
+	id, ok := parseUUID(raw)
+	if !ok {
+		writeErr(w, http.StatusBadRequest, "bad_request", "invalid seed_track id")
+		return
+	}
+	q := dbq.New(h.pool)
+	track, err := q.GetTrackByID(r.Context(), id)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			writeErr(w, http.StatusNotFound, "not_found", "seed_track not found")
+			return
+		}
+		h.logger.Error("api: get radio seed track failed", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		return
+	}
+	album, err := q.GetAlbumByID(r.Context(), track.AlbumID)
+	if err != nil {
+		h.logger.Error("api: get radio seed album failed", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		return
+	}
+	artist, err := q.GetArtistByID(r.Context(), track.ArtistID)
+	if err != nil {
+		h.logger.Error("api: get radio seed artist failed", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		return
+	}
+	writeJSON(w, http.StatusOK, RadioResponse{
+		Tracks: []TrackRef{trackRefFrom(track, album.Title, artist.Name)},
+	})
+}