feat(audio-cpp): AUDIOCPP_DEFAULT_BACKEND fallback for models without a backend option

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Models whose options carry no explicit backend: open their session on the
CPU backend even in accelerator images. The gallery entries carry
backend:best since #11892; this covers hand-written model configurations
the same way, per deployment: the environment variable supplies the
fallback, an explicit backend: option always wins (merged beside the
existing threads and maingpu fallbacks), and validation reuses the
option parser.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Plamen K. Kosseff <p.kosseff@gmail.com>

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