feat(observability): add structured application logging

Emit correlated JSON logs from the API and FunASR services, propagate request IDs to the client, and configure bounded opt-in Docker logging.

Document the host-wide Grafana/Loki query workflow and keep observability deployment ownership outside the application repository.

server/Dockerfile

@@ -28,6 +28,8 @@ RUN --mount=type=cache,id=teaching-feedback-cargo-registry,target=/usr/local/car
 FROM debian:bookworm-slim AS runtime
 
 ARG DEBIAN_MIRROR=http://deb.debian.org
+ARG APP_VERSION=0.2.0
+ARG GIT_SHA=unknown
 
 RUN sed -i "s|http://deb.debian.org|${DEBIAN_MIRROR}|g" /etc/apt/sources.list.d/debian.sources \
     && apt-get update \
@@ -44,7 +46,11 @@ COPY --from=builder /build/output/migrate-user-owner /usr/local/bin/migrate-user
 ENV HOST=0.0.0.0 \
     PORT=8080 \
     AUDIO_STORAGE_DIR=/data/audio \
-    RUST_LOG=info
+    RUST_LOG=info \
+    APP_ENV=lan \
+    APP_VERSION=${APP_VERSION} \
+    GIT_SHA=${GIT_SHA} \
+    LOG_FORMAT=json
 
 USER app