S2S: In Undo handler for sharer inbox, support unfollowing sharer tickets

src/Vervis/Federation/Offer.hs

@@ -545,11 +545,26 @@ sharerUndoF shr =
         getRecip
         personInbox
         personFollowers
-        (\ _ _ -> return $ Just "Undo object is a RemoteFollow, but isn't under this sharer")
+        tryTicket
     where
     getRecip = do
         sid <- getKeyBy404 $ UniqueSharer shr
         getBy404 $ UniquePersonIdent sid
+    tryTicket pid fsid = do
+        mltid <- getKeyBy $ UniqueLocalTicketFollowers fsid
+        case mltid of
+            Nothing -> return $ Just "Undo object is a RemoteFollow, but isn't under this sharer"
+            Just ltid -> do
+                mtal <- getBy $ UniqueTicketAuthorLocal ltid
+                case mtal of
+                    Just (Entity talid tal)
+                        | ticketAuthorLocalAuthor tal == pid -> do
+                            mtup <- getBy $ UniqueTicketUnderProjectAuthor talid
+                            return $
+                                case mtup of
+                                    Nothing -> Nothing
+                                    Just _ -> Just "Undo object is a RemoteFollow of a ticket authored by this sharer, but is hosted by the project"
+                    _ -> return $ Just "Undo object is a RemoteFollow of a ticket of another author"
 
 projectUndoF
     :: ShrIdent