Synchronize connected accounts twice a day

One named syncInterval replaces the two 24-hour literals in the scheduler: a
successful run now waits 12 hours, and an account becomes due again 12 hours
after its last successful sync. Two background fetches a day stay inside Enable
Banking's documented allowance of roughly four per account per day, which a
failing sync's hourly retries also draw from, so the bank rate limits that
prompted this are no more likely than before.

OPERATIONS.txt

@@ -275,7 +275,8 @@ errors are displayed separately from expired consent.
 
 Only booked transactions are persisted. Each new account initially requests the
 selected number of calendar months (12 by default); the bank may provide less.
-Daily sync deliberately overlaps each account's last successful sync by 14 days.
+Automatic sync runs twice a day, every 12 hours from the last successful run,
+and deliberately overlaps each account's last successful sync by 14 days.
 Per-account cursors prevent another account's recent sync from skipping a new
 account's history. Reconnection preserves the saved history choice and existing
 cursors. Changing the choice or reconnecting does not backfill already-synced