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

README.md

@@ -41,7 +41,7 @@ Copy `.env.example` to `.env` and configure:
 | `HTTPS_PROXY`       | empty              | Outbound HTTPS proxy             |
 | `DB_PATH`           | `data/uptodown.db` | SQLite database path             |
 | `DOWNLOAD_DIR`      | `/tmp/uptodown`    | Temporary download directory     |
-| `MAX_FILE_SIZE_MB`  | `2000`             | Maximum upload file size in MB   |
+| `MAX_FILE_SIZE_MB`  | `50`               | Maximum upload file size in MB   |
 | `COOKIES_FILE`      | empty              | Path to cookies.txt for auth     |
 | `TZ`                | `UTC`              | Timezone                         |
 | `BOT_ALLOWED_USERS` | empty              | Comma-separated allowed user IDs |