default conf: lower expire days from 2 weeks to 1

Rationale: the former quite long grace period was due mainly to the long update
period of codesearch, which has now been improved. One week should be enough to
account for the update lags of other services.

References:
- https://people.debian.org/~stapelberg/2014/12/03/debian-code-search-instant.html
- https://lists.debian.org/debian-qa/2014/12/msg00034.html