Replace the YatteeServerAPI setAuthHeader/buildHeaders pattern (which was
race-prone on the shared actor across multiple instances) with a generic
mechanism: HTTPClient now supports a defaultHeaders dictionary applied to
every request, and ContentService builds a per-instance HTTPClient with the
basic-auth Authorization header baked in whenever credentials are configured.
The same code path now works uniformly for Invidious, Piped, PeerTube, and
Yattee Server, so any instance sitting behind a reverse proxy that requires
HTTP Basic Auth can be authenticated regardless of backend type. Cached
default API actors are still reused when no basic-auth header is needed.
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.
