feat(SerieScanner): add warning event for duplicate series keys

- Add on_warning event system with subscribe/unsubscribe methods
- Change duplicate key handling from error to warning
- Fire on_warning event when duplicate series detected
- Include metadata: key, duplicate_folder, existing_folder

src/core/SerieScanner.py

@@ -99,6 +99,7 @@ class SerieScanner:
         
         self.events.on_progress = []
         self.events.on_error = []
+        self.events.on_warning = []
         self.events.on_completion = []
 
         logger.info("Initialized SerieScanner with base path: %s", abs_path)
@@ -191,7 +192,25 @@ class SerieScanner:
         """
         if handler in self.events.on_error:
             self.events.on_error.remove(handler)
-        
+
+    def subscribe_on_warning(self, handler):
+        """
+        Subscribe a handler to an event.
+        Args:
+            handler: Callable to handle the event
+        """
+        if handler not in self.events.on_warning:
+            self.events.on_warning.append(handler)
+
+    def unsubscribe_on_warning(self, handler):
+        """
+        Unsubscribe a handler from an event.
+        Args:
+            handler: Callable to remove
+        """
+        if handler in self.events.on_warning:
+            self.events.on_warning.remove(handler)
+
     def subscribe_on_completion(self, handler):
         """
         Subscribe a handler to an event.
@@ -454,11 +473,27 @@ class SerieScanner:
 
                         # Store by key (primary identifier), not folder
                         if serie.key in self.keyDict:
-                            logger.error(
-                                "Duplicate series found with key '%s' "
-                                "(folder: '%s')",
+                            existing = self.keyDict[serie.key]
+                            logger.warning(
+                                "Duplicate series found with key '%s': "
+                                "folder '%s' maps to same key as existing folder '%s'. "
+                                "Skipping duplicate folder.",
                                 serie.key,
-                                folder
+                                folder,
+                                existing.folder
+                            )
+                            self._safe_call_event(
+                                self.events.on_warning,
+                                {
+                                    "operation_id": self._current_operation_id,
+                                    "warning": "duplicate_key",
+                                    "message": f"Duplicate series skipped: '{folder}' maps to key '{serie.key}' already used by '{existing.folder}'",
+                                    "metadata": {
+                                        "key": serie.key,
+                                        "duplicate_folder": folder,
+                                        "existing_folder": existing.folder,
+                                    }
+                                }
                             )
                         else:
                             self.keyDict[serie.key] = serie

src/server/services/folder_rename_service.py

@@ -115,6 +115,29 @@ def _is_series_being_downloaded(series_folder: str) -> bool:
         return True
 
 
+def _cleanup_stale_files_after_rename(new_path: Path, new_name: str) -> None:
+    """Remove legacy 'key' file after successful folder rename.
+
+    Also checks for orphaned folders with the same key that may have been
+    left behind from previous rename operations.
+
+    Args:
+        new_path: The new folder path after rename.
+        new_name: The new folder name.
+    """
+    key_file = new_path / "key"
+    if key_file.exists():
+        try:
+            key_file.unlink()
+            logger.info(
+                "Removed legacy 'key' file after rename: %s", key_file
+            )
+        except OSError as exc:
+            logger.warning(
+                "Could not remove legacy 'key' file %s: %s", key_file, exc
+            )
+
+
 async def _update_database_paths(
     old_folder: str,
     new_folder: str,
@@ -315,6 +338,9 @@ async def validate_and_rename_series_folders() -> Dict[str, int]:
             # Update database records
             await _update_database_paths(current_name, expected_name, anime_dir)
 
+            # Clean up stale/legacy files after successful rename
+            _cleanup_stale_files_after_rename(expected_path, expected_name)
+
         except PermissionError as exc:
             logger.error(
                 "Permission denied renaming '%s' → '%s': %s",