fix(cast): put the relay's progress log behind the same switch

Most of what the relay says repeats without bound — a line every fiftieth
fragment for the length of a film, one per dropped fragment whenever a client
falls behind — and it lands in the terminal the app was started from. Those go
through `debuglog` now, restored with:

    NODE_DEBUG=cast-relay npm start

The loss of fMP4 framing stays on console.warn. It recovers by rescanning, so
nothing throws and no other part of the system hears about it; that line is the
only trace that the picture on the television is missing a piece. Every other
failure here throws, and the renderer already reports the rejection.

The logging guard now covers both cast modules, each with the one line it is
meant to keep audible.

Also corrects the header comment, which still claimed CORS was granted on the
subtitle path only. It has been on both since the stream needed it too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UGY17EPph5LsLzePPXhUVc

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