Renames YatteeServerCredentialsManager → BasicAuthCredentialsManager so the
same Keychain-backed username/password storage can be reused for any instance
type that sits behind a reverse proxy requiring HTTP Basic Auth. Adds a
one-time migration that moves existing items from the legacy
'com.yattee.yatteeserver' Keychain service to 'com.yattee.basicauth',
preserving the iCloud-sync attribute. No behavior change for end users.
Videos that haven't premiered yet were triggering repeated notifications
on every background refresh cycle. Filter them out by checking isUpcoming
flag and rejecting videos with future publish dates.
Also decode isUpcoming/premiereTimestamp from Yattee Server feed responses
instead of hardcoding false/nil.