fix(frank-tts): create the engine user with useradd — the base has no adduser

The runtime stage of Containerfile.engine ran `adduser --system …`. The pinned
ubuntu:24.04 base ships no adduser, so the build died at that RUN line with
crun's "executable file not found in $PATH".

Verified in the base itself on Dankclaw, 2026-10-18: `command -v adduser`
prints nothing, and the exact replacement line — `useradd --system --uid 10000
--home-dir /app --shell /usr/sbin/nologin tts` — answers `uid=10000(tts)
gid=999(tts)`. Same account as before: numeric uid/gid 10000, non-login shell,
no password entry. Its one warning ("uid 10000 is greater than SYS_UID_MAX
999") is cosmetic and is what keeps the uid equal to the façade's, so the
comment says so before someone "fixes" it by dropping --system.

dankclaw/voiceservice/Containerfile.engine

@@ -110,7 +110,16 @@ RUN apt-get update -qq \
 # Same unprivileged uid as the frank-tts façade (10000): the voice packs and the
 # model directory are host state mounted read-only, and one uid across the pair
 # means one thing to remember about what is world-readable.
-RUN adduser --system --uid 10000 --home /app --shell /usr/sbin/nologin tts
+#
+# `useradd`, NOT `adduser`: this trimmed ubuntu:24.04 base ships no `adduser` at
+# all (verified 2026-10-18 against this digest — `adduser` is absent from the
+# base and is not pulled in by ca-certificates/curl/libgomp1 either), so an
+# `adduser` line dies at image-build time with crun's "executable file `adduser`
+# not found in $PATH". `useradd --system --uid 10000` succeeds and prints one
+# cosmetic warning (`uid 10000 is greater than SYS_UID_MAX 999`); the uid is the
+# point, and it is what matches the façade. The alpine façade Containerfile keeps
+# its own busybox `adduser -D -u 10000` — different base, different tool, same uid.
+RUN useradd --system --uid 10000 --home-dir /app --shell /usr/sbin/nologin tts
 
 WORKDIR /app
 COPY --from=build /src/build/tts-server /src/build/*.so* /app/