refactor: Add context manager support to ImageDownloader

- Add __aenter__ and __aexit__ methods
- Add close() method for session cleanup
- Add retry_delay parameter for testability
- Add session attribute (currently unused, for future optimization)

Note: Current implementation still creates per-request sessions.
Tests that mock session attribute will need updating to match
actual session-per-request pattern or implementation needs
refactoring to use persistent session.

docs/task3_status.md

@@ -128,7 +128,7 @@ The unit tests were written based on assumptions about the API that don't match
 
 -   `src/core/utils/nfo_generator.py`: ✅ 19/19 tests passing
 -   `src/core/utils/image_downloader.py`: ⚠️ 12/20 tests passing (8 require refactoring for context manager/attribute access)
--   `src/core/services/tmdb_client.py`: ⚠️ 0/16 tests passing (async mocking challenges)  
+-   `src/core/services/tmdb_client.py`: ⚠️ 0/16 tests passing (async mocking challenges)
 -   `src/core/services/nfo_service.py`: ✅ 4 tests for update logic passing
 
 **Integration Tests:**

src/core/utils/image_downloader.py

@@ -27,17 +27,25 @@ class ImageDownloadError(Exception):
 class ImageDownloader:
     """Utility for downloading and validating images.
     
+    Supports async context manager protocol for proper resource cleanup.
+    
     Attributes:
         max_retries: Maximum retry attempts for downloads
         timeout: Request timeout in seconds
         min_file_size: Minimum valid file size in bytes
+        session: Optional aiohttp session (managed internally)
+    
+    Example:
+        >>> async with ImageDownloader() as downloader:
+        ...     await downloader.download_poster(url, path)
     """
     
     def __init__(
         self,
         max_retries: int = 3,
         timeout: int = 60,
-        min_file_size: int = 1024  # 1 KB
+        min_file_size: int = 1024,  # 1 KB
+        retry_delay: float = 1.0
     ):
         """Initialize image downloader.
         
@@ -45,10 +53,28 @@ class ImageDownloader:
             max_retries: Maximum retry attempts
             timeout: Request timeout in seconds
             min_file_size: Minimum valid file size in bytes
+            retry_delay: Delay between retries in seconds
         """
         self.max_retries = max_retries
         self.timeout = timeout
         self.min_file_size = min_file_size
+        self.retry_delay = retry_delay
+        self.session: Optional[aiohttp.ClientSession] = None
+    
+    async def __aenter__(self):
+        """Enter async context manager."""
+        return self
+    
+    async def __aexit__(self, exc_type, exc_val, exc_tb):
+        """Exit async context manager and cleanup resources."""
+        await self.close()
+        return False
+    
+    async def close(self):
+        """Close aiohttp session if open."""
+        if self.session and not self.session.closed:
+            await self.session.close()
+            self.session = None
     
     async def download_image(
         self,