fix(tmdb): harden aiohttp session lifecycle

- Add async context manager to NFOService wrapping TMDBClient + ImageDownloader
- Add TMDBClient.__del__ warning when session leaks
- Log exc_info on session recreation for traceback visibility
- Document async-with usage in docs/DEVELOPMENT.md and docs/TESTING.md
- Add unit tests covering leak detection, context-manager cleanup, and connector-closed warning

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/DEVELOPMENT.md

@@ -114,3 +114,32 @@ def download_service(mock_anime_service, mock_queue_repository):
 ```
 
 9. Troubleshooting Development Issues
+
+### Async Context Managers for aiohttp
+
+All `aiohttp.ClientSession` usages must be wrapped in `async with`:
+
+```python
+# Correct — session properly closed on exit
+async with TMDBClient(api_key="key") as client:
+    result = await client.search_tv_show("Show")
+
+# Wrong — session may leak if exception occurs
+client = TMDBClient(api_key="key")
+result = await client.search_tv_show("Show")
+await client.close()  # May not be called if exception raised earlier
+```
+
+**Why:**
+- `aiohttp.ClientSession` holds TCP connections that must be explicitly closed
+- If exception occurs before `close()`, session leaks
+- Context manager guarantees `__aexit__` runs even on exceptions
+
+**Services that use aiohttp:**
+- `TMDBClient` — has `__aenter__`/`__aexit__`, use `async with`
+- `ImageDownloader` — has `__aenter__`/`__aexit__`, use `async with`
+- `NFOService` — wraps both above, use `async with`
+
+**Verification:**
+- Missing context manager usage triggers `__del__` warning on garbage collection
+- Integration tests verify no "Unclosed client session" errors in logs