the image agrees with itself about the timezone (V-545)

compose set TZ=Europe/Samara and Go read it, so clock replies and quiet
hours were already local. But /etc/localtime in the image pointed at
Etc/UTC, so a caller asking the system zone instead of the environment
answered UTC. The reminder path shells out to python dateparser, which is
such a caller.

TZ is now a build arg on the runtime stage. It points the symlink, writes
/etc/timezone and sets ENV TZ, so the image is local on its own. Compose
passes the zone it already declares, so the zone stays written in one
place.

Dockerfile

@@ -82,11 +82,22 @@ FROM debian:trixie-slim AS runtime
 # tzdata so the TZ env (set in compose) resolves — otherwise Go can't load the
 # zone and time.Now() stays UTC, and mavend answers clock/date queries and
 # evaluates quiet-hours in UTC.
+#
+# TZ is a build arg as well as an env because the image was self-inconsistent
+# without it (V-545): compose set TZ=Europe/Samara and Go read it, but
+# /etc/localtime still pointed at Etc/UTC, so anything asking the system zone
+# instead of the environment answered UTC. The reminder path shells out to
+# python dateparser, which is exactly such a caller. Compose passes the same
+# zone it already declares, so the zone is written in one place.
+ARG TZ=Etc/UTC
+ENV TZ=$TZ
 RUN apt-get update && apt-get install -y --no-install-recommends \
       ca-certificates libvulkan1 mesa-vulkan-drivers libgomp1 tzdata \
       python3 python3-pip && \
     pip3 install --no-cache-dir --break-system-packages 'dateparser==1.4.1' && \
     apt-get purge -y --auto-remove python3-pip && \
+    ln -snf "/usr/share/zoneinfo/$TZ" /etc/localtime && \
+    echo "$TZ" > /etc/timezone && \
     rm -rf /var/lib/apt/lists/*
 
 # runtime native libs: whisper/ggml (incl. vulkan) are real files in deps/lib.