stacks/index-tts: own FastAPI wrapper for IndexTTS-2 + deploy playbook

Adds a third TTS to the irv-ml1 fleet. IndexTTS-2 is Bilibili's
emotion-controllable zero-shot TTS (paper 2506.21619). Distinguishing
capability vs the existing two: timbre and emotion are disentangled —
clone a voice's timbre from one reference and the emotion from a
different reference, OR set emotion via 8-vector, OR derive it from a
text description. Neither CosyVoice 3 nor Qwen3-TTS-1.7B-Base does
this cleanly in English.

Wrapper is owned end-to-end (~150 lines in app.py) — the only existing
FastAPI fork (csllpr/index-tts-fastapi) targets v1 and is a dormant
single-commit repo. Upstream IndexTTS-2 ships only a Gradio webui.

Layout follows the qwen3-tts pattern:
  stacks/index-tts/
    Dockerfile           — CUDA 12.8 base, IndexTTS pinned to a SHA
    app.py               — FastAPI: POST /v1/audio/speech + /v1/voices
    entrypoint.sh        — one-time HF snapshot_download of the weights
    compose.yaml         — env-driven, GPU pinning support, bind mounts
    .env.example         — port 8192, fp16, paths
    README.md            — API examples + comparison vs the other TTS
  playbooks/deploy-index-tts.yaml  — elway playbook for irv-ml1

Voice and emotion libraries are flat host dirs of WAVs, bind-mounted.
Drop a new <name>.wav and /v1/voices picks it up immediately.

License caveat: IndexTTS-2 weights ship under a custom Bilibili
license (free at our scale, not OSI-open). README documents it.

stacks/index-tts/entrypoint.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+# entrypoint.sh — fetch IndexTTS-2 weights on first run, then exec server.
+#
+# IndexTTS2() expects checkpoint files pre-staged under model_dir; if
+# they're missing, the constructor sys.exit(1)s. Pull them via
+# huggingface_hub.snapshot_download on first start. ~5-7 GB; subsequent
+# starts skip the download because config.yaml is the gate.
+
+set -e
+
+MODEL_DIR="${INDEX_TTS_MODEL_DIR:-/app/checkpoints}"
+mkdir -p "${MODEL_DIR}"
+
+if [ ! -f "${MODEL_DIR}/config.yaml" ]; then
+    echo "[index-tts] downloading IndexTTS-2 weights to ${MODEL_DIR} (~5-7 GB, one-time)"
+    python3 - <<EOF
+from huggingface_hub import snapshot_download
+snapshot_download(
+    repo_id="IndexTeam/IndexTTS-2",
+    local_dir="${MODEL_DIR}",
+)
+EOF
+    echo "[index-tts] download complete"
+fi
+
+exec "$@"