GetPreview keyed only on StreamingMode + JWT, so a disabled channel or one
with no playout was declared Available and then failed confusingly (404 from
IptvController, or an indefinitely-blocking manifest request). Extend it to
take isEnabled + playoutCount and check JWT, then disabled, then no-playout,
before falling back to the existing mode-based rules; order is documented
in a comment. Also corrects a stale comment in ChannelPreviewResponseModel.cs
that claimed a C# string generates a TypeScript union (it does not).
timothy
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