Run the reconciliation pass at startup and once a day (closes #250)

The pass makes a database query for every cached file and ran on every
eviction tick, so on a large cache one pass could outlast the tick and
the next started at once. It now runs on a timer of its own that starts
when a pass ends, a day long. Eviction and the removal of expired
negative cache rows keep the eviction interval.

Model: opus-5-5

README.md

@@ -168,7 +168,7 @@ references rather than using filesystem refcounting.
 Source and transformed image files, and the `.meta` file beside each transformed
 image, are written under a temporary name, synced to disk, and only then renamed
 into place, so a crash leaves no file under its own name with part of its data.
-When pixa starts, and every five minutes after, it removes each cached source or
+When pixa starts, and once a day after, it removes each cached source or
 transformed image whose file is empty, or not the size the database records for
 it, with a warning naming the file; the next request for it fetches the source
 or converts the image again.