Start no eviction pass after a stop, and end a pass quietly

Both pass runners do nothing once the loop's context is cancelled, so
a stop no longer starts an eviction pass after a cancelled
reconciliation or on a pending write-pressure wakeup or tick. In
evictBatch, a candidate that fails after cancellation ends the batch
with the context's error instead of logging its own warning; this
replaces the check before each candidate, since every candidate
started after cancellation fails at its first database call. A stop
now logs at most one warning.

Model: opus-5-5

TODO.md

@@ -32,7 +32,8 @@ P2: security: referer blacklist
 - 2026-10-04 shutdown stops cache eviction in progress (closes #102):
   `StartEviction` runs the eviction goroutine with its own context, which
   `StopEviction` cancels, so a pass in progress stops at its next database
-  call, file, row or eviction candidate instead of running to completion;
+  call, file, row or eviction candidate instead of running to completion, and
+  no pass starts after it, so a stop logs at most one warning;
   `StopEviction` takes a context and, when that context ends before the
   goroutine exits, stops waiting and returns its error; the handlers' stop hook
   passes fx's stop context, so an eviction still running when fx's stop