Add Download Queue Management & Concurrent Downloads (#231)

## Summary

This PR completely overhauls the download system to support concurrent
downloads, cancellation, and advanced queue
management. No more waiting for stuck downloads to block the entire
queue!

  ## New Features

  ###  **Concurrent Downloads**
  - **3 simultaneous downloads** by default (configurable)
- ThreadPoolExecutor-based architecture replaces single-threaded
bottleneck
  - Downloads no longer block each other

  ###  **Download Cancellation**
  - **Cancel button** for active/queued downloads
  - Clean cancellation with partial file cleanup
  - Thread-safe cancellation flags

  ### **Queue Priority & Reordering**
  - **Priority-based queue** (lower number = higher priority)
  - **Editable priority inputs** in UI for queued items
  - **Bulk reordering** API support

  ## UI Enhancements

- **Enhanced status table** with Priority, Progress, and Actions columns
  - **Progress bars** for active downloads
  - **Cancel buttons** for downloads
- **Queue management controls**: Refresh, Clear Completed, Active
Counter
  - **Real-time updates** every 60 seconds

## Implementation

  ### New Architecture
  - `queue.PriorityQueue` replaces simple `set()` for proper ordering
  - `concurrent.futures.ThreadPoolExecutor` handles worker threads
  - `threading.Event` flags enable clean cancellation
  - Thread-safe operations with proper locking

  ### New API Endpoints
  DELETE /api/download/{id}/cancel     # Cancel downloads
PUT /api/queue/{id}/priority # Set priorityPOST /api/queue/reorder #
Bulk reorder
  GET /api/downloads/active          # Active tracking
  DELETE /api/queue/clear            # Clear completed

  ### New Environment Variables
  ```bash
  MAX_CONCURRENT_DOWNLOADS=3          # Worker thread count
  DOWNLOAD_PROGRESS_UPDATE_INTERVAL=5  # Progress frequency
  ```

### Fully backward compatible - existing functionality unchanged

### Fixes

  - Resolves: Downloads blocking when one gets stuck
  - Resolves: No way to cancel problematic downloads
  - Resolves: No queue management or prioritization
  
  #183

---------

Co-authored-by: CaliBrain <calibrain@l4n.xyz>

env.py

@@ -35,6 +35,8 @@ else:
     LOG_LEVEL = os.getenv("LOG_LEVEL", "INFO").upper()
 ENABLE_LOGGING = string_to_bool(os.getenv("ENABLE_LOGGING", "true"))
 MAIN_LOOP_SLEEP_TIME = int(os.getenv("MAIN_LOOP_SLEEP_TIME", "5"))
+MAX_CONCURRENT_DOWNLOADS = int(os.getenv("MAX_CONCURRENT_DOWNLOADS", "3"))
+DOWNLOAD_PROGRESS_UPDATE_INTERVAL = int(os.getenv("DOWNLOAD_PROGRESS_UPDATE_INTERVAL", "5"))
 DOCKERMODE = string_to_bool(os.getenv("DOCKERMODE", "false"))
 _CUSTOM_DNS = os.getenv("CUSTOM_DNS", "").strip()
 USE_DOH = string_to_bool(os.getenv("USE_DOH", "false"))