feat(server): server-controlled default streaming TTS model

The streaming speech provider settings page was seeding its model
picker with a hardcoded `volcengine/seed-tts-2.0`, which contradicts
the just-landed "streaming model catalog comes from the server"
contract. Now the default also comes from configKV.

- `UNSPEECH_UPSTREAM.streaming` gains `defaultModel?: string`.
- `GET /api/v1/audio/models/streaming` response gains `default: string
  | null` reading that field.
- admin slice apply: `streaming.defaultModel` survives key/URL
  rotation alongside `streaming.models`.
- `providerOfficialSpeechStreaming.listModels` stashes the value in a
  module-scope ref, exposed via `getDefaultStreamingModel()`.
- Streaming speech settings page reads the helper instead of the
  hardcoded id; falls back to the first server-returned model if the
  operator hasn't curated a default. `handleGenerateSpeech` no longer
  has a `seed-tts-2.0` backstop — a missing backend prefix in the
  selected model id now throws instead of silently defaulting.