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/domain/constants.go

@@ -6,7 +6,7 @@ const (
 	RateLimitInterval      = 1 * time.Second
 	ProgressThreshold      = 5.0
 	MaxFileSizeEnvKey      = "MAX_FILE_SIZE_MB"
-	DefaultMaxFileSize     = 2000
+	DefaultMaxFileSize     = 50
 	DefaultDailyQuotaMB    = 100
 	DefaultWeeklyQuotaMB   = 500
 	DefaultMonthlyQuotaMB  = 2000