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>

src/core/services/tmdb_client.py

@@ -173,7 +173,11 @@ class TMDBClient:
                     last_error = e
                     # If connector/session was closed, try to recreate it
                     if "Connector is closed" in str(e) or isinstance(e, AttributeError):
-                        logger.warning("Session issue detected, recreating session: %s", e)
+                        logger.warning(
+                            "Session issue detected, recreating session: %s",
+                            e,
+                            exc_info=True,
+                        )
                         self.session = None
                         await self._ensure_session()
 
@@ -339,6 +343,14 @@ class TMDBClient:
             await self.session.close()
             self.session = None
             logger.debug("TMDB client session closed")
+
+    def __del__(self):
+        """Warn if session is unclosed during garbage collection."""
+        if self.session is not None and not self.session.closed:
+            logger.warning(
+                "TMDBClient: unclosed session detected. "
+                "Use 'async with TMDBClient(...)' or call close() explicitly."
+            )
     
     def clear_cache(self):
         """Clear the request cache."""