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/playlists_foryou_refresh.go

@@ -0,0 +1,99 @@
+package api
+
+import (
+	"errors"
+	"net/http"
+	"time"
+
+	"github.com/jackc/pgx/v5"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/auth"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/playlists"
+)
+
+// foryouRefreshResp is the wire shape for POST
+// /api/playlists/system/for-you/refresh. track_ids is the playlist's
+// tracks in playlist position order — the frontend enqueues this
+// list directly to start playback without a second roundtrip.
+//
+// playlist_id is null + track_ids empty when the build succeeded but
+// the user's library yielded no eligible candidates (degenerate
+// empty-library case).
+type foryouRefreshResp struct {
+	PlaylistID *string  `json:"playlist_id"`
+	TrackCount int32    `json:"track_count"`
+	TrackIDs   []string `json:"track_ids"`
+}
+
+// handleForYouRefresh re-runs BuildSystemPlaylists synchronously for
+// the calling user and returns their freshly-built For-You playlist.
+// Used by the home Playlists row's For-You tile play button: click
+// triggers refresh, the response carries the track IDs, the
+// frontend enqueues + auto-plays.
+//
+// Authenticated user only — each user refreshes only their own
+// For-You.
+func (h *handlers) handleForYouRefresh(w http.ResponseWriter, r *http.Request) {
+	user, ok := auth.UserFromContext(r.Context())
+	if !ok {
+		writeErr(w, http.StatusUnauthorized, "auth_required", "")
+		return
+	}
+	if err := playlists.BuildSystemPlaylists(r.Context(), h.pool, h.logger, user.ID, time.Now(), h.dataDir); err != nil {
+		h.logger.Error("for-you refresh: build failed",
+			"user_id", uuidToString(user.ID), "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "build failed")
+		return
+	}
+
+	q := dbq.New(h.pool)
+	sysVariant := "for_you"
+	pl, err := q.GetSystemPlaylistByVariantForUser(r.Context(),
+		dbq.GetSystemPlaylistByVariantForUserParams{
+			UserID:        user.ID,
+			SystemVariant: &sysVariant,
+		})
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			// Build succeeded but no For-You row — empty library.
+			writeJSON(w, http.StatusOK, foryouRefreshResp{
+				PlaylistID: nil,
+				TrackCount: 0,
+				TrackIDs:   []string{},
+			})
+			return
+		}
+		h.logger.Error("for-you refresh: lookup failed",
+			"user_id", uuidToString(user.ID), "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		return
+	}
+
+	// Fetch tracks in playlist position order so the frontend can
+	// enqueue them directly.
+	rows, err := q.ListPlaylistTracks(r.Context(), pl.ID)
+	if err != nil {
+		h.logger.Error("for-you refresh: list tracks failed",
+			"playlist_id", uuidToString(pl.ID), "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "list tracks failed")
+		return
+	}
+	trackIDs := make([]string, 0, len(rows))
+	for _, row := range rows {
+		// Skip rows where the track was removed from the library
+		// (LiveTrackID won't be Valid). The snapshot row still
+		// exists in playlist_tracks, but there's nothing to enqueue.
+		if !row.LiveTrackID.Valid {
+			continue
+		}
+		trackIDs = append(trackIDs, uuidToString(row.LiveTrackID))
+	}
+
+	idStr := uuidToString(pl.ID)
+	writeJSON(w, http.StatusOK, foryouRefreshResp{
+		PlaylistID: &idStr,
+		TrackCount: pl.TrackCount,
+		TrackIDs:   trackIDs,
+	})
+}