fix(audiobooks): add /libraries/{id}/playlists for mobile create modal

The ABS mobile create-playlist modal loads existing playlists
BEFORE opening the form, hitting:
    GET /api/libraries/{libraryId}/playlists
and reading `data.results` (NOT `data.playlists`). It then iterates
each playlist's `items[]` to render "already in this playlist"
badges. silo's existing `/api/playlists` route emitted the wrong
envelope (`{playlists: [...]}`) and was at the wrong path; the
modal silently treated the response as empty, the user saw an
empty existing-playlist list, and depending on the device-side
UX (no "Create New" CTA when results was undefined) the create
action looked dead.

Add handleListLibraryPlaylists at `/libraries/{libraryId}/playlists`,
emit `{results: [Playlist full-shape]}` so playlist.items is
populated for the membership check. libraryId param accepted but
ignored — silo scopes playlists per (user, profile) globally.

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

internal/audiobooks/abs/handler.go

@@ -369,6 +369,10 @@ func (h *Handler) mountRoutes(r chi.Router) {
 			// Playlists — owner-gated CRUD with cross-user public reads,
 			// realtime events on every mutation, batch endpoints.
 			r.Get(prefix+"/playlists", h.handleListPlaylists)
+			// Per-library playlist list — mobile create-playlist modal
+			// loads from here before opening the form. Emits
+			// `{results: [...]}` with full-shape entries.
+			r.Get(prefix+"/libraries/{libraryId}/playlists", h.handleListLibraryPlaylists)
 			r.Post(prefix+"/playlists", h.handleCreatePlaylist)
 			r.Get(prefix+"/playlists/{id}", h.handleGetPlaylist)
 			r.Patch(prefix+"/playlists/{id}", h.handleUpdatePlaylist)

internal/audiobooks/abs/playlists_handler.go

@@ -158,6 +158,47 @@ func (h *Handler) playlistItems(r *http.Request, playlistID string) []map[string
 // uniformly with the collections handler's chiURLID.
 func playlistURLID(r *http.Request) string { return chi.URLParam(r, "id") }
 
+// handleListLibraryPlaylists — GET /libraries/{libraryId}/playlists.
+//
+// The ABS mobile create-playlist modal hits this endpoint BEFORE
+// opening the form so it can show "already in playlist X" badges and
+// the existing-playlists picker. It accesses `data.results` on the
+// response (NOT `data.playlists`), and iterates `playlist.items` to
+// check membership, so we emit:
+//   {"results": [Playlist full-shape with items[]]}
+//
+// The libraryId URL param is accepted but ignored — silo scopes
+// playlists per (user, profile) globally rather than per-library;
+// the mobile UI only needs to know which of the user's playlists
+// already contain the selected item.
+//
+// Ref: audiobookshelf-app components/modals/playlists/AddCreateModal.vue
+// loadPlaylists().
+func (h *Handler) handleListLibraryPlaylists(w http.ResponseWriter, r *http.Request) {
+	a, ok := absAuthFrom(r)
+	if !ok || a.UserID == "" {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+	if h.deps.PlaylistStore == nil {
+		writeJSON(w, http.StatusOK, map[string]any{"results": []any{}})
+		return
+	}
+	rows, err := h.deps.PlaylistStore.ListUserPlaylists(r.Context(), a.UserID, a.ProfileID)
+	if err != nil {
+		slog.Error("abs library playlist list failed", "err", err, "user", a.UserID)
+		http.Error(w, "playlist list failed", http.StatusInternalServerError)
+		return
+	}
+	out := make([]map[string]any, 0, len(rows))
+	for _, p := range rows {
+		// Full-shape (with items[]) so the modal can check
+		// existing-membership via playlist.items.some(...).
+		out = append(out, h.playlistFullShape(r, p))
+	}
+	writeJSON(w, http.StatusOK, map[string]any{"results": out})
+}
+
 // handleListPlaylists — GET /playlists.
 // Returns the caller's playlists wrapped in {"playlists": [...]}.
 // List-shape (no items[]).