Add Fish Audio as a third voice engine (cloned voices, streams PCM, no ffmpeg)

[BigBeedle]

What this adds

Fish Audio (fish.audio) as a third voice engine, beside Kokoro and ElevenLabs. The draw: Fish Audio lets anyone clone their own voice (or build a character voice) in their account and drive it by API — so an agent can literally speak in the voice its person made for it. Engine order when enabled: fish → elevenlabs → Kokoro.

Why it fits this codebase

• Degrade, never mute — the fish path is wrapped in the exact same try/fallback shape as ElevenLabs; any cloud failure logs one line and falls back to the configured Kokoro voice.
• No new dependencies. Fish Audio streams format: "pcm" (16-bit mono at the configured sample_rate), so chunks go from httpx.stream straight into the existing playback path. Unlike the ElevenLabs route there is no ffmpeg involved. Uses only httpx + numpy, both already in the tree.
• Key handling mirrors _get_elevenlabs_key — macOS Keychain (item backtalk-fish, renameable via fish.key_slot), Linux secret-tool, FISH_AUDIO_API_KEY env var as the Windows last resort. Never in a file.
• Backwards compatible — mouth reads the block with .get, so a backtalk.json written before this engine existed keeps working unchanged; the fish block ships disabled in DEFAULTS.

Config

"fish": {
  "enabled": true,
  "reference_id": "<your voice model id on fish.audio>",
  "model": "s2.1-pro-free"
}

The default model is s2.1-pro-free deliberately: it works on an unfunded account. The paid tiers (s1, s2-pro, s2.1-pro) return 402 Payment Required until the account holds credits — the DEFAULTS comment says so explicitly, so nobody debugs a 402 blind. The person's reference_id is listable via GET https://api.fish.audio/model?self=true.

Tested

Windows 11, real account, real cloned voice: config parses, sentences stream and play through the long-lived output stream (audio laws respected — the engine only feeds PCM into the existing _play_stream path), interrupt behavior unchanged, and the Kokoro fallback fires correctly on both a missing key and an unfunded model tier (402). macOS/Linux keychain lookups follow the ElevenLabs code path verbatim but I could only exercise the env-var branch — worth one audition on a Mac before merge.

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