engines: import f5-tts + kokoro + tortoise sidecars into the tree

The python FastAPI sidecars have lived ad-hoc at /srv/appdata/
<engine>/build/ on the host without version control. Bringing them into
the skald repo so the engine code travels with the cross-engine
routing it depends on.

This commit lands the VANILLA version of each engine on main:

  engines/f5-tts/    SWivid F5-TTS (CC-BY-NC weights flagged)
  engines/kokoro/    hexgrad Kokoro-82M (Apache 2.0 top to bottom)
  engines/tortoise/  neonbjb Tortoise-TTS (Apache 2.0 top to bottom)

Engine-specific kludges (question doubling, GPU coordination,
pause-duration tuning) get layered on engine/* branches per the
README. Main stays the safe-to-read baseline.

engines/kokoro/Dockerfile

@@ -0,0 +1,35 @@
+# Sulkta build of Kokoro-82M TTS.
+#
+# License: Apache 2.0 (code AND model weights). Clean stack — no
+# CC-BY-NC asterisk like F5-TTS's Emilia weights. This is the
+# narrator engine for sleep-quality audiobook reads; F5-TTS stays
+# around for voice-cloning cases.
+#
+# Kokoro is small enough to run on CPU but we use the cuda base
+# anyway to stay consistent with f5-tts and so it'll pick up the
+# GPU when no other tenant has it.
+FROM pytorch/pytorch:2.6.0-cuda12.4-cudnn9-runtime
+
+ENV DEBIAN_FRONTEND=noninteractive \
+    PYTHONUNBUFFERED=1 \
+    HF_HOME=/cache/hf \
+    HF_HUB_DISABLE_TELEMETRY=1
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        ffmpeg \
+        espeak-ng \
+        ca-certificates \
+        curl \
+    && rm -rf /var/lib/apt/lists/*
+
+# kokoro pulls phonemizer + soundfile + espeakng transitively.
+RUN pip install --no-cache-dir 'kokoro>=0.9.0' 'fastapi>=0.115.0' 'uvicorn>=0.32.0' 'soundfile>=0.13.0'
+
+RUN mkdir -p /cache/hf /audio
+
+COPY kokoro_server.py /app/kokoro_server.py
+WORKDIR /app
+
+EXPOSE 7860
+
+CMD ["uvicorn", "kokoro_server:app", "--host", "0.0.0.0", "--port", "7860"]