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/f5-tts/Dockerfile

@@ -0,0 +1,41 @@
+# Sulkta build of F5-TTS — upstream ghcr.io/swivid/f5-tts:main was
+# shipped with torch 2.11/torchaudio 2.4 ABI mismatch on 2026-05-13,
+# breaking import torchaudio at boot. We rebuild on a known-good
+# pytorch base + pip install f5-tts.
+#
+# Image tag in localhost-registry: registry.example.local:5000/f5-tts:<ver>
+#
+# License: Apache 2.0 (code) / CC-BY-NC (Emilia-trained weights).
+# Personal use OK; redistribution gray-area — flagged.
+
+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 \
+        git \
+        ca-certificates \
+        curl \
+    && rm -rf /var/lib/apt/lists/*
+
+# Base torch 2.6.0 + torchaudio 2.6.0; f5-tts pulls a recent
+# transformers (5.x) which needs torch >=2.5's modern
+# torch.library.custom_op type signatures.
+RUN pip install --no-cache-dir 'f5-tts>=1.0.0'
+
+# Pre-warm the HF cache directory.
+RUN mkdir -p /cache/hf /audio /voices
+
+COPY f5_server.py /app/f5_server.py
+WORKDIR /app
+
+EXPOSE 7860
+
+# Skald talks to our purpose-built FastAPI server, not Gradio.
+# Models load at startup (first request would otherwise pay the
+# cold-start cost). uvicorn on :7860 to keep the port stable.
+CMD ["uvicorn", "f5_server:app", "--host", "0.0.0.0", "--port", "7860"]