fix cloud library scale and playback compatibility

Root cause: display-only cloud library filtering was reused by scan jobs, merged cloud mounts could be skipped, and OpenList listing relied on WebDAV/first-page behavior that could cap huge directories around 100 items. Cloud scans also let new items consume probe budget before existing rows with missing track/artwork metadata.\n\nChanges:\n- split display filtering from scannable cloud filtering so merged cloud mounts still scan\n- add OpenList API pagination with WebDAV fallback\n- prioritize existing cloud media missing metadata before new imports\n- restrict automatic cloud sync to one successful 19:00-21:00 daily window and keep manual scan immediate\n- disable startup cloud full-scan by default, with an explicit opt-in setting\n- improve Emby/cloud playback compatibility and cache/search/test coverage from the continued work

internal/handler/stats_extra.go

@@ -134,7 +134,7 @@ func statsLibrariesHandler(svc *service.Container) gin.HandlerFunc {
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 			return
 		}
-		libs = service.FilterShadowedCloudLibraries(libs)
+		libs = service.FilterDisplayCloudLibraries(c.Request.Context(), svc.Repo, libs)
 		out := make([]gin.H, 0, len(libs))
 		visibility := mediaVisibilityForRequest(c, svc)
 		for _, l := range libs {