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/tortoise/Dockerfile

@@ -0,0 +1,45 @@
+# Sulkta build of Tortoise-TTS.
+#
+# Voice roster (built-in, no cloning needed): angie, daniel, deniro,
+# emma, freeman, geralt, halle, jlaw, lj, mol, myself, pat, pat2,
+# rainbow, snakes, tim_reynolds, tom, train_atkins, train_dotrice,
+# train_dreams, train_grace, train_kennard, train_lescault,
+# train_mouse, weaver, william. ~26 voices baked in.
+#
+# License: Apache 2.0 (code) + Apache 2.0 (model weights). Clean
+# stack for share/publish.
+#
+# Speed: slow. Trade for quality. Standard preset is ~10x slower
+# than Kokoro; high_quality is ~30x slower. Worth it for the
+# audiobook-quality bar.
+
+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 \
+    TORTOISE_MODELS_DIR=/cache/tortoise-models
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        ffmpeg \
+        git \
+        ca-certificates \
+        curl \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN pip install --no-cache-dir \
+        'tortoise-tts>=3.0.0' \
+        'fastapi>=0.115.0' \
+        'uvicorn>=0.32.0' \
+        'soundfile>=0.13.0' \
+        'numpy<2'
+
+RUN mkdir -p /cache/hf /cache/tortoise-models /audio
+
+COPY tortoise_server.py /app/tortoise_server.py
+WORKDIR /app
+
+EXPOSE 7860
+
+CMD ["uvicorn", "tortoise_server:app", "--host", "0.0.0.0", "--port", "7860"]