feat(audiobooks): add PlaylistStore interface + ABS envelope helper

Defines the storage contract and wire-shape serialiser the playlists
handlers will consume. Envelope test asserts the eight (or nine with
coverPath) top-level keys including description always round-tripped
correctly and coverPath omitted when empty.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/audiobooks/abs/playlists.go

@@ -0,0 +1,67 @@
+package abs
+
+import (
+	"context"
+	"time"
+)
+
+// PlaylistStore is the narrow slice of abs_playlists + abs_playlist_items
+// the playlists handlers need. Implemented by ABSPlaylistStore in
+// internal/audiobooks/abs_playlist_store.go.
+type PlaylistStore interface {
+	ListUserPlaylists(ctx context.Context, userID, profileID string) ([]Playlist, error)
+	GetPlaylist(ctx context.Context, id string) (Playlist, error)
+	CreatePlaylist(ctx context.Context, p Playlist) error
+	UpdatePlaylist(ctx context.Context, p Playlist) error
+	DeletePlaylist(ctx context.Context, id string) error
+	ListPlaylistItems(ctx context.Context, playlistID string) ([]PlaylistItem, error)
+	AddPlaylistItem(ctx context.Context, playlistID, libraryItemID, episodeID string) error
+	RemovePlaylistItem(ctx context.Context, playlistID, libraryItemID, episodeID string) error
+}
+
+// Playlist is the in-memory representation of an abs_playlists row.
+type Playlist struct {
+	ID          string
+	UserID      string
+	ProfileID   string
+	Name        string
+	Description string
+	CoverItem   string // empty when unset
+	IsPublic    bool
+	CreatedAt   time.Time
+	UpdatedAt   time.Time
+}
+
+// PlaylistItem is the in-memory representation of an abs_playlist_items row.
+type PlaylistItem struct {
+	PlaylistID    string
+	LibraryItemID string
+	EpisodeID     string // empty for audiobook items
+	Position      int
+	AddedAt       time.Time
+}
+
+// playlistToABS shapes a Playlist in the ABS wire format. When items
+// is nil the list-shape is emitted (no "items" key); when items is
+// non-nil (possibly empty) the full-shape is emitted.
+//
+// coverPath is omitted when CoverItem is empty (matches continuum).
+// Description is always present (round-tripped from storage).
+func playlistToABS(p Playlist, items []map[string]any) map[string]any {
+	out := map[string]any{
+		"id":          p.ID,
+		"userId":      p.UserID,
+		"name":        p.Name,
+		"description": p.Description,
+		"isPublic":    p.IsPublic,
+		"createdAt":   p.CreatedAt.UnixMilli(),
+		"lastUpdate":  p.UpdatedAt.UnixMilli(),
+	}
+	if p.CoverItem != "" {
+		out["coverPath"] = p.CoverItem
+	}
+	if items != nil {
+		out["items"] = items
+	}
+	return out
+}

internal/audiobooks/abs/playlists_envelope_test.go

@@ -0,0 +1,61 @@
+package abs
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+	"time"
+)
+
+// TestPlaylistEnvelope_HasRequiredKeys asserts the eight (or nine with
+// coverPath) top-level keys are present when populated. coverPath is
+// emitted only when non-empty.
+func TestPlaylistEnvelope_HasRequiredKeys(t *testing.T) {
+	now := time.Date(2026, 5, 26, 12, 0, 0, 0, time.UTC)
+	out := playlistToABS(Playlist{
+		ID:          "01HPL",
+		UserID:      "1",
+		Name:        "queue",
+		Description: "",
+		CoverItem:   "01HCOVER",
+		IsPublic:    false,
+		CreatedAt:   now,
+		UpdatedAt:   now,
+	}, []map[string]any{})
+	body, _ := json.Marshal(out)
+	js := string(body)
+	for _, key := range []string{
+		`"id":`, `"userId":`, `"name":`, `"description":`,
+		`"isPublic":`, `"coverPath":`, `"createdAt":`, `"lastUpdate":`, `"items":`,
+	} {
+		if !strings.Contains(js, key) {
+			t.Errorf("envelope missing %s; got %s", key, js)
+		}
+	}
+	if out["coverPath"] != "01HCOVER" {
+		t.Errorf("coverPath = %v, want 01HCOVER", out["coverPath"])
+	}
+}
+
+// TestPlaylistEnvelope_OmitsCoverPathWhenEmpty asserts cover_item=""
+// produces no coverPath key (matches continuum).
+func TestPlaylistEnvelope_OmitsCoverPathWhenEmpty(t *testing.T) {
+	out := playlistToABS(Playlist{
+		ID: "01HPL", UserID: "1", Name: "x",
+		CreatedAt: time.Now(), UpdatedAt: time.Now(),
+	}, []map[string]any{})
+	if _, has := out["coverPath"]; has {
+		t.Errorf("coverPath emitted when empty: %v", out)
+	}
+}
+
+// TestPlaylistListShape_OmitsItems asserts nil items produces no items key.
+func TestPlaylistListShape_OmitsItems(t *testing.T) {
+	out := playlistToABS(Playlist{
+		ID: "01HPL", UserID: "1", Name: "x",
+		CreatedAt: time.Now(), UpdatedAt: time.Now(),
+	}, nil)
+	if _, has := out["items"]; has {
+		t.Errorf("list-shape includes items key (should be detail-only): %v", out)
+	}
+}