feat(crawler): CRAWLER_CHROMIUM_BINARY to use system chromium (0.45.0)

Skips the chromiumoxide fetcher when CRAWLER_CHROMIUM_BINARY is set,
unblocking Linux_arm64 deployments (Raspberry Pi 5) where the
fetcher's upstream snapshot bucket has no reliable build. The
Dockerfile gains an INSTALL_CHROMIUM build-arg that adds
chromium-headless-shell + fonts-liberation to the runtime image when
set; default off so cloud/x86 images stay slim.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/Dockerfile

@@ -21,8 +21,23 @@ RUN touch src/main.rs src/lib.rs && cargo build --locked --release
 FROM debian:bookworm-slim
 # `curl` is for the container HEALTHCHECK; `ca-certificates` is for
 # outbound HTTPS (crawler covers/pages).
+#
+# INSTALL_CHROMIUM is an opt-in for deployments that can't use the
+# chromiumoxide fetcher path (notably Linux_arm64 / Raspberry Pi, where
+# the upstream snapshot bucket has no usable build). When `true`, adds
+# Debian's apt-packaged headless chromium plus a baseline font set —
+# pair with `CRAWLER_CHROMIUM_BINARY=/usr/bin/chromium-headless-shell`
+# at runtime so the launcher uses it. Default `false` keeps cloud/x86
+# images slim.
+#
+# Build the Pi image with:
+#   docker compose build --build-arg INSTALL_CHROMIUM=true backend
+ARG INSTALL_CHROMIUM=false
 RUN apt-get update \
  && apt-get install -y --no-install-recommends ca-certificates curl \
+ && if [ "$INSTALL_CHROMIUM" = "true" ]; then \
+      apt-get install -y --no-install-recommends chromium-headless-shell fonts-liberation; \
+    fi \
  && rm -rf /var/lib/apt/lists/*
 
 # Non-root runtime user. The API binary doesn't need any root