refactor: remove container selection, extract helpers, add logging and tests

- Remove container picker UI, keyboard, settings, and ContainerOptions
  entirely — yt-dlp handles container format automatically
- Remove Container field from DownloadJob struct and related references
- Extract buildFormatList helper to deduplicate format list building
  between handleURLInput and handleBackStep
- Break runDownload into trackDownloadProgress + handleDownloadCompletion
- Add HTTP timeout (30s) to downloadFile helper
- Log applyQuota and AddHistory errors instead of discarding
- Log nil stepState warnings in all handler entry points
- Include job.StderrLog in download failure messages
- Add tests for buildFormatList, findFormatByID, determineMediaType,
  and buildFormatString
- Update README with search, DRM fallback, and quota feature docs
- Remove unused strings import from keyboard.go

internal/handler/handler.go

@@ -280,7 +280,7 @@ func (h *Handler) routePrefixedCallback(ctx context.Context, chatID int64, msgID
 	case strings.HasPrefix(data, "lang_"):
 		h.handleLanguageSelection(ctx, chatID, msgID, userID, data[5:])
 	case strings.HasPrefix(data, "container_"):
-		h.handleContainerSelection(ctx, chatID, msgID, userID, data[10:])
+		// container selection was removed — yt-dlp handles it
 	case strings.HasPrefix(data, "pl_page_"):
 		h.handlePlaylistPage(ctx, chatID, msgID, userID, data[8:])
 	case strings.HasPrefix(data, "pl_entry_"):
@@ -295,8 +295,6 @@ func (h *Handler) routePrefixedCallback(ctx context.Context, chatID int64, msgID
 		}
 	case data == "back_quality":
 		h.handleSettingsSelection(ctx, chatID, msgID, userID, "quality")
-	case data == "back_container":
-		h.handleSettingsSelection(ctx, chatID, msgID, userID, "container")
 	case data == "back_settings":
 		h.backToSettings(ctx, chatID, msgID, userID)
 	case strings.HasPrefix(data, "settings_set_"):
@@ -573,6 +571,7 @@ func (h *Handler) historyCallback(ctx context.Context, chatID int64, msgID int,
 func (h *Handler) applyQuota(userID int64, fileSize int64) {
 	q, err := h.Repo.GetOrCreateQuotas(userID)
 	if err != nil {
+		slog.Error("get quotas for apply", "user_id", userID, "error", err)
 		return
 	}
 	q = h.resetQuotasIfNeeded(q)
@@ -583,7 +582,9 @@ func (h *Handler) applyQuota(userID int64, fileSize int64) {
 	q.DailyUsedMB += sizeMB
 	q.WeeklyUsedMB += sizeMB
 	q.MonthlyUsedMB += sizeMB
-	_ = h.Repo.UpdateQuotas(userID, q)
+	if err := h.Repo.UpdateQuotas(userID, q); err != nil {
+		slog.Error("update quotas", "user_id", userID, "error", err)
+	}
 }
 
 // resetQuotasIfNeeded resets quota counters if the period has changed.