refactor: split mutex, add download queue, wire context cancellation

- Split single sync.Mutex into per-map mutexes (activeJobs, stepStates,
  rateLimiter, pendingInput, cancelFuncs) to reduce contention
- Add download semaphore (channel-buffered, 3 concurrent) as worker pool
- Add cancelFuncs map with context cancellation wired into download
  goroutines for clean shutdown
- Replace playlist busy-poll (500ms sleep loop) with synchronous
  runDownload + Done channel for completion tracking
- Use full UUID for job IDs instead of 8-char prefix
- Download thumbnail URL to temp file before sending (InputFileUpload)
- Change MAX_FILE_SIZE_MB default from 2000 to 50
- Clean up cancel funcs after normal download completion

internal/handler/playlist.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"fmt"
 	"strconv"
-	"time"
 
 	"uptodownBot/internal/domain"
 )
@@ -164,6 +163,8 @@ func (h *Handler) handlePlaylistConfirm(ctx context.Context, chatID int64, msgID
 			}
 		}
 
+		dlCtx, dlCancel := context.WithCancel(context.Background())
+
 		job := &domain.DownloadJob{
 			ID:             h.generateJobID(),
 			UserID:         userID,
@@ -176,19 +177,17 @@ func (h *Handler) handlePlaylistConfirm(ctx context.Context, chatID int64, msgID
 			Status:         domain.StatusDownloading,
 			Title:          entry.Title,
 			FileSize:       format.Filesize,
+			Done:           make(chan struct{}),
 		}
 
 		h.setActiveJob(userID, job)
-		h.runDownload(context.Background(), job)
+		h.setCancelFunc(userID, dlCancel)
 
-		// Wait for active job to complete
-		for {
-			j := h.getActiveJob(userID)
-			if j == nil || j.Status == domain.StatusCompleted || j.Status == domain.StatusFailed || j.Status == domain.StatusCancelled {
-				break
-			}
-			time.Sleep(500 * time.Millisecond)
-		}
+		h.runDownload(dlCtx, job)
+
+		// runDownload is synchronous — it blocks until the download completes.
+		// The Done channel is closed inside runDownload when it exits.
+		// No busy-polling needed.
 	}
 
 	h.sendText(ctx, chatID, "All playlist downloads completed.")