kokoro: persist custom voices across container recreate

Wrapper only enumerates one voice directory (settings.voices_dir,
default /app/api/src/voices/v1_0 — inside the container's writable
layer, not bind-mounted). Override via VOICES_DIR=/app/user_voices
(host bind mount) and add a command shim that cp -r's built-ins from
the in-image v1_0 into user_voices on every start. Built-ins re-seed
fresh from the image (so upgrades that add voices propagate); custom
.pt files in user_voices are preserved (cp -r is additive).

Also adds scripts/blend_kokoro_voice.py + a playbook around it that
mirrors the wrapper's request-time voice="a(w)+b(w)" math but writes
the result as a named .pt to user_voices, making it discoverable via
GET /v1/audio/voices and persistent across recreate. Defaults to
athena = af_bella(2)+af_aoede(1) normalized.

stacks/kokoro/compose.yaml

@@ -27,18 +27,30 @@ services:
     # that env var harmlessly; the gpu variant honors it.
     ports:
       - "${KOKORO_BIND:-0.0.0.0}:${KOKORO_PORT}:8880"
+    # Persistent custom voices: the wrapper only enumerates one voice
+    # directory (controlled by VOICES_DIR; default in-image is
+    # /app/api/src/voices/v1_0 which is NOT bind-mounted). We point it
+    # at /app/user_voices (which IS host-bind-mounted) and seed the
+    # built-ins into it on every container start. Custom voices written
+    # by scripts/blend_kokoro_voice.py land directly on the host bind
+    # mount and survive `up --force-recreate` and image upgrade.
+    # `cp -r` (no `-n`): always refresh built-ins from the image so
+    # upgrades that add/change built-in voicepacks propagate.
+    # Custom-named .pt files in user_voices that don't exist in v1_0
+    # are NOT touched (cp -r only copies entries from src).
+    command: ["/bin/bash", "-c", "cp -r /app/api/src/voices/v1_0/. /app/user_voices/ && exec ./entrypoint.sh"]
     environment:
       - NVIDIA_VISIBLE_DEVICES=${KOKORO_GPU_DEVICES:-}
       - USE_GPU=${KOKORO_USE_GPU:-false}
       - API_LOG_LEVEL=${KOKORO_LOG_LEVEL:-INFO}
+      - VOICES_DIR=/app/user_voices
     volumes:
-      # Optional voice-overlay mount — drop a custom <name>.pt into the
-      # host dir to make it available alongside the 60+ built-ins. The
-      # image already ships voicepacks at this path, so the bind mount
-      # SHADOWS them — only do this if you actually want to manage the
-      # full voice library yourself. For most deploys, leave the mount
-      # commented out and use the in-image voices.
-      # - ${KOKORO_VOICES_DIR}:/app/api/src/voices/v1_0
+      # Host-bind-mounted voice directory. Hosts both the image's
+      # built-ins (re-seeded on each container start by the command
+      # shim above) and any custom voices created via the blend
+      # script. Must be chowned to uid 1001 (appuser inside the
+      # container) on the host so the shim's cp can write — handled
+      # by playbooks/deploy-kokoro.yaml.
       - ${KOKORO_USER_VOICES_DIR}:/app/user_voices
     healthcheck:
       # The image is python-based with curl available. /v1/audio/voices