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>

.env.example

@@ -74,6 +74,14 @@ CRAWLER_DOWNLOAD_ALLOWLIST=
 CRAWLER_ALLOW_ANY_HOST=false
 # Hard cap on a single image body. Default 32 MiB.
 CRAWLER_MAX_IMAGE_BYTES=33554432
+# Path to a system Chromium binary. When set, the crawler skips the
+# bundled-fetcher download. Required on platforms without a usable
+# upstream Chromium build (notably Linux_arm64 / Raspberry Pi). On
+# Debian: /usr/bin/chromium-headless-shell or /usr/bin/chromium. On
+# Ubuntu the package is chromium-browser (different path). Pair with
+# `docker compose build --build-arg INSTALL_CHROMIUM=true backend` so
+# the image actually contains the binary.
+CRAWLER_CHROMIUM_BINARY=
 
 # ----- Frontend -----
 # The frontend container runs SvelteKit's Node adapter on :3000 and