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.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CL9zvnnvcp1mvA8o7impxk

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".