fix: call pg_notification_queue_usage() automatically when the LISTEN channel bug is detected (#4858)

There's a PostgreSQL bug that doesn't let any listener to register in the DB:
https://www.postgresql.org/message-id/flat/CAK98qZ3wZLE-RZJN_Y%2BTFjiTRPPFPBwNBpBi5K5CU8hUHkzDpw%40mail.gmail.com
The only workaround is to advance the async notification queue tail,
which can be done by executing:  "SELECT pg_notification_queue_usage();".
Before we just logged a HINT with this suggestion, but now we call that function directly
and then let the listener to automatically recover.

No automated tests were added here as it would be too complex and this is a PostgreSQL bug. But this was manually tested following the steps on https://github.com/PostgREST/postgrest/pull/4581#issuecomment-3690610592

src/PostgREST/Observation.hs

@@ -38,7 +38,7 @@ data Observation
   | DBListenStart (Maybe ByteString) (Maybe ByteString) Text Text -- host, port, version string, channel
   | DBListenFail Text (Either SQL.ConnectionError SomeException)
   | DBListenRetry Int
-  | DBListenBugHint -- https://github.com/PostgREST/postgrest/issues/3147
+  | DBListenBugCallQueryFix
   | DBListenerGotSCacheMsg ByteString
   | DBListenerGotConfigMsg ByteString
   | DBListenerConnectionCleanupFail SomeException