Create shared ContactsFixtures module with globally-unique qso_timestamp
seconds to prevent unique-constraint violations when async test modules
run in parallel and insert contacts with identical dedup-key columns.
The qso_timestamp column is timestamp(0), so microsecond offsets were
truncated — use System.unique_integer monotonic seconds instead.
Also make count-asserting tests resilient to sandbox-leaked contacts
from prior tests by using >= assertions or status-based checks rather
than exact counts.
Includes automated DateTime.add → DateTime.shift migration from
mix format.
