smp-server: fix notification store key retention leak

deleteExpiredNtfs trimmed each notifier's message list but never removed
the outer NtfStore map key, so one empty entry per notifier queue that
ever received a notification was retained forever (grows with the active
notifier set, never shrinks).

Remove the outer key when its list becomes empty, and make storeNtf fully
atomic so it cannot race the removal and write a notification to an
orphaned TVar. Verified with the load bench (ntfexp): after expiry
ntfStore_keys drops from the queue count to 0 instead of staying flat.

src/Simplex/Messaging/Server/NtfStore.hs

@@ -33,13 +33,10 @@ data MsgNtf = MsgNtf
   }
 
 storeNtf :: NtfStore -> NotifierId -> MsgNtf -> IO ()
-storeNtf (NtfStore ns) nId ntf = do
-  TM.lookupIO nId ns >>= atomically . maybe newNtfs (`modifyTVar'` (ntf :))
+storeNtf (NtfStore ns) nId ntf =
   -- TODO [ntfdb] coalesce messages here once the client is updated to process multiple messages
   -- for single notification.
-  -- when (isJust prevNtf) $ incStat $ msgNtfReplaced stats
-  where
-    newNtfs = TM.lookup nId ns >>= maybe (TM.insertM nId (newTVar [ntf]) ns) (`modifyTVar'` (ntf :))
+  atomically $ TM.lookup nId ns >>= maybe (TM.insertM nId (newTVar [ntf]) ns) (`modifyTVar'` (ntf :))
 
 deleteNtfs :: NtfStore -> NotifierId -> IO Int
 deleteNtfs (NtfStore ns) nId = atomically (TM.lookupDelete nId ns) >>= maybe (pure 0) (fmap length . readTVarIO)
@@ -48,18 +45,17 @@ deleteExpiredNtfs :: NtfStore -> Int64 -> IO Int
 deleteExpiredNtfs (NtfStore ns) old =
   foldM (\expired -> fmap (expired +) . expireQueue) 0 . M.keys =<< readTVarIO ns
   where
-    expireQueue nId = TM.lookupIO nId ns >>= maybe (pure 0) expire
-    expire v = readTVarIO v >>= \case
-      [] -> pure 0
-      _ ->
-        atomically $ readTVar v >>= \case
-          [] -> pure 0
-          -- check the last message first, it is the earliest
-          ntfs | systemSeconds (ntfTs $ last $ ntfs) < old -> do
+    expireQueue nId = atomically $ TM.lookup nId ns >>= maybe (pure 0) (expire nId)
+    expire nId v = readTVar v >>= \case
+      [] -> TM.delete nId ns >> pure 0
+      -- check the last message first, it is the earliest
+      ntfs
+        | systemSeconds (ntfTs $ last ntfs) < old -> do
             let !ntfs' = filter (\MsgNtf {ntfTs = ts} -> systemSeconds ts >= old) ntfs
-            writeTVar v ntfs'
-            pure $! length ntfs - length ntfs'
-          _ -> pure 0
+            if null ntfs'
+              then TM.delete nId ns >> pure (length ntfs)
+              else writeTVar v ntfs' >> pure (length ntfs - length ntfs')
+        | otherwise -> pure 0
 
 data NtfLogRecord = NLRv1 NotifierId MsgNtf