Bound the work-queue durability claim to the appendfsync window

Redis runs with appendfsync everysec, so an unclean stop loses up to a
second of journaled entries. Name that where the docstrings promised a
crash costs nothing, and state the setting and its trade-off on the
configuration page.

backend/fluksio/flow/pipeline.py

@@ -1522,7 +1522,8 @@ class Pipeline:
 
         With a work queue attached the event is journaled and the caller
         returns immediately — that is the path every external trigger takes, so
-        a crash mid-cascade loses nothing. Interactive callers (a manual run, a
+        a crash mid-cascade costs at most the last second of journaled work
+        rather than the whole wave. Interactive callers (a manual run, a
         draft preview) pass ``durable=False`` and get the old synchronous
         behaviour, because they are waiting for the result.
 

backend/fluksio/flow/queue.py

@@ -5,6 +5,10 @@ poll — becomes a work item before anything runs. The item is journaled first
 and acknowledged only once the wave it started has quiesced, so an engine that
 dies mid-cascade picks the work up again on the way back rather than losing it.
 
+Redis journals with ``appendfsync everysec``, so the bound is the last second:
+an unclean stop of Redis or of its host loses up to a second of entries. An
+engine that dies while Redis lives loses none of them.
+
 Two implementations: Redis Streams, which is what makes the above true, and an
 in-memory one for tests and for running without Redis, where "durable" degrades
 honestly to "not".