fix(auth): hand over library and media item ownership on admin deletion

When an admin was deleted, the ON DELETE SET NULL foreign key would set
created_by_admin_id to NULL on all their libraries. This caused the scanner
to fail to find an admin ID for broadcasting scan-complete WebSocket messages.

- On admin deletion, reassign all libraries and media items to the next admin
- Prevents created_by_admin_id from ever being NULL on active libraries
- Uses new ReassignLibraries and ReassignMediaItems DB queries

internal/handlers/auth.go

@@ -903,6 +903,22 @@ func (h *AuthHandler) DeleteUser(c *echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "cannot delete the last admin account"})
 	}
 
+	// If deleting an admin, reassign their libraries and media items to another admin
+	// before deletion to prevent ON DELETE SET NULL from orphaning ownership
+	if targetUserRole == "admin" {
+		successor, err := h.db.GetFirstAdminExclude(c.Request().Context(), targetUserUUID)
+		if err == nil {
+			_ = h.db.ReassignLibraries(c.Request().Context(), database.ReassignLibrariesParams{
+				CreatedByAdminID:   targetUserUUID,
+				CreatedByAdminID_2: successor.ID,
+			})
+			_ = h.db.ReassignMediaItems(c.Request().Context(), database.ReassignMediaItemsParams{
+				AddedByAdminID:   targetUserUUID,
+				AddedByAdminID_2: successor.ID,
+			})
+		}
+	}
+
 	// Delete user (this will cascade to delete all related data)
 	err = h.db.DeleteUser(c.Request().Context(), targetUserUUID)
 	if err != nil {