feat(api): /api/playlists* handlers for M7 #352 slice 1

9 handlers covering create / get / list / update / delete / append /
remove / reorder / cover. Permissions enforced in the service layer
(ErrForbidden -> 403 not_authorized) so the handler is a thin
HTTP-shape adapter. /cover serves the cached collage from disk via
http.ServeFile; 404 when cover_path is nil.

Server gained a DataDir field so the playlists service can find the
collage cache; api.Mount picks up two new params (playlistsSvc and
dataDir). The stale TestRoutesRegisteredInMount Mount() call in
library_test.go was missing the tracks.Service argument added by an
earlier slice — fixed in passing.

Wire codes follow the project's existing nested errorBody envelope:
not_found, not_authorized, unauthenticated, bad_request, server_error.
The plan called for a flat envelope, but the api package's writeErr
already produces {"error":{"code":"...","message":"..."}} and
deviating here would break every existing client.

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

internal/api/library_test.go

@@ -443,7 +443,7 @@ func TestRoutesRegisteredInMount(t *testing.T) {
 	r := chi.NewRouter()
 	w := playevents.NewWriter(h.pool, slog.New(slog.NewTextHandler(io.Discard, nil)),
 		30*time.Minute, 0.5, 30000)
-	Mount(r, h.pool, h.logger, w, config.RecommendationConfig{RadioSize: 50, RadioSizeMax: 200, RecentlyPlayedHours: 1}, h.lidarrCfg, h.lidarrRequests, h.lidarrQuarantine)
+	Mount(r, h.pool, h.logger, w, config.RecommendationConfig{RadioSize: 50, RadioSizeMax: 200, RecentlyPlayedHours: 1}, h.lidarrCfg, h.lidarrRequests, h.lidarrQuarantine, h.tracks, h.playlists, h.dataDir)
 
 	paths := []string{
 		"/api/artists",