fix(docker): bump Chromium pin to 156.0.8078.73 (#1614)

Debian has a newer Chromium in trixie-security (156.0.8078.73), so this
moves the pin from .47 to .73 and the snapshot to 20261029T020000Z, the
first one that has it for both amd64 and arm64. Same shape as #1601.

To check the bypasser still works on it, I started a throwaway container
from the current `dev` image, upgraded Chromium to .73 the way the
Dockerfile does, and ran `internal_bypasser.get()` against example.com.
It starts, loads the page and returns it, the same as on .47. I also
pointed both versions at the Cloudflare demo challenge page at
scrapingcourse.com. They behave the same there, but neither one solves
it from where I ran it (same challenge detected 4 times, then abort), so
that only shows no difference, not a working solve.

I also ran it against an Anna's Archive search page on both versions.
Both returned the full results page, but the log shows no bypass
attempts, so that mirror didn't serve a challenge from where I ran it
and the solver itself wasn't exercised.

@calibrain your call on if we should update to this, it is minor bump to
what has already been pushed.

Dockerfile

@@ -148,9 +148,9 @@ ENTRYPOINT ["/usr/bin/dumb-init", "--"]
 
 FROM base AS shelfmark
 
-# --- Chromium (PINNED to 156.0.8078.47) ---
-ARG CHROMIUM_VERSION=156.0.8078.47-1~deb13u1
-ARG CHROMIUM_SNAPSHOT=20261023T000000Z
+# --- Chromium (PINNED to 156.0.8078.73) ---
+ARG CHROMIUM_VERSION=156.0.8078.73-1~deb13u1
+ARG CHROMIUM_SNAPSHOT=20261029T020000Z
 
 RUN echo "deb [check-valid-until=no] https://snapshot.debian.org/archive/debian-security/${CHROMIUM_SNAPSHOT}/ trixie-security main" \
         > /etc/apt/sources.list.d/chromium-pin-snapshot.list && \