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/server/server.go

@@ -23,6 +23,7 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/lidarrquarantine"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/lidarrrequests"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/playevents"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/playlists"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/subsonic"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/tracks"
 	"git.fabledsword.com/bvandeusen/minstrel/web"
@@ -62,6 +63,11 @@ type Server struct {
 	SubsonicCfg       subsonic.Config
 	EventsCfg         config.EventsConfig
 	RecommendationCfg config.RecommendationConfig
+	// DataDir is the on-disk root for cached artifacts (currently
+	// playlist cover collages under <DataDir>/playlist_covers/). Empty
+	// strings are tolerated by tests that don't exercise persisted-cover
+	// codepaths; production callers should pass a writable directory.
+	DataDir string
 }
 
 func New(logger *slog.Logger, pool *pgxpool.Pool, scanner ScanTrigger, subCfg subsonic.Config, eventsCfg config.EventsConfig, recCfg config.RecommendationConfig) *Server {
@@ -98,7 +104,8 @@ func (s *Server) Router() http.Handler {
 		lidarrReqs := lidarrrequests.NewService(s.Pool, lidarrCfg, lidarrClientFn, nil)
 		lidarrQuar := lidarrquarantine.NewService(s.Pool, lidarrCfg, lidarrClientFn)
 		tracksSvc := tracks.NewService(s.Pool, s.Logger, lidarrUnmonitorAdapter{fn: lidarrClientFn})
-		api.Mount(r, s.Pool, s.Logger, writer, s.RecommendationCfg, lidarrCfg, lidarrReqs, lidarrQuar, tracksSvc)
+		playlistsSvc := playlists.NewService(s.Pool, s.Logger, s.DataDir)
+		api.Mount(r, s.Pool, s.Logger, writer, s.RecommendationCfg, lidarrCfg, lidarrReqs, lidarrQuar, tracksSvc, playlistsSvc, s.DataDir)
 		// /api/admin/scan is the only admin route owned by the server package
 		// (it needs the Scanner). Register it as a single inline-middleware
 		// route — using r.Route("/api/admin", ...) here would create a second