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

cmd/bot/main.go

@@ -29,7 +29,7 @@ func main() {
 	token := os.Getenv("BOT_TOKEN")
 	dbPath := getEnvDefault("DB_PATH", "data/uptodown.db")
 	downloadDir := getEnvDefault("DOWNLOAD_DIR", "/tmp/uptodown")
-	maxFileSizeMB := getEnvIntDefault("MAX_FILE_SIZE_MB", 2000)
+	maxFileSizeMB := getEnvIntDefault("MAX_FILE_SIZE_MB", 50)
 	cookiesFile := os.Getenv("COOKIES_FILE")
 
 	if token == "" {