Require explicit http.Client in podcast service (DIP)

Remove the silent nil-fallback in NewPodcastService /
NewPodcastServiceWithLogger that fabricated a default http.Client when
callers passed nil. The service now panics on nil, forcing callers to
inject their own client and own the HTTP timeout / transport policy
explicitly. DefaultHTTPClientTimeout remains exported so production
wiring (cmd/player/main.go) keeps a sensible default at the composition
root, not buried in the service.

Updated TestPodcastService_NilHTTPClient_Defaults to
TestPodcastService_NilHTTPClient_Panics to document the new contract.
All production call sites already pass a non-nil client.

Refs task 8a.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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