feat: implement download queue service with persistence, priority, and retry logic

- Added comprehensive download queue service (download_service.py)
  - Priority-based queue management (HIGH, NORMAL, LOW)
  - Concurrent download processing with configurable limits
  - Automatic queue persistence to JSON file
  - Retry logic for failed downloads with max retry limits
  - Real-time progress tracking and WebSocket broadcasting
  - Queue operations: add, remove, reorder, pause, resume
  - Statistics tracking: download speeds, sizes, ETA calculations

- Created comprehensive unit tests (test_download_service.py)
  - 23 tests covering all service functionality
  - Tests for queue management, persistence, retry logic
  - Broadcast callbacks, error handling, and lifecycle

- Added structlog dependency for structured logging
- Updated infrastructure.md with download service documentation
- Removed completed task from instructions.md

All tests passing (23/23)

requirements.txt

@@ -8,6 +8,7 @@ python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4
 aiofiles==23.2.1
 websockets==12.0
+structlog==24.1.0
 pytest==7.4.3
 pytest-asyncio==0.21.1
 httpx==0.25.2