feat: wire headless-shell sidecar for challenge-gated polling

Also bumps backend/Dockerfile's build stage to golang:1.26-alpine —
chromedp v0.16.0 and cdproto both require go 1.26, and the pinned
1.24-alpine base no longer builds the module.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.env.example

@@ -52,3 +52,7 @@ WEB_PASSWORD=
 # BATCH x (COOLDOWN / INTERVAL) series hold the cooldown cadence — 84 with these
 # defaults. Beyond that the cadence stretches uniformly rather than breaking;
 # raise BATCH or lower INTERVAL. Keep BATCH x STAGGER under INTERVAL.
+
+# Headless-shell CDP endpoint for sites behind a JavaScript challenge (kagane).
+# Unset disables browser polling; those sites then rely on the userscript alone.
+BROWSER_WS_URL=ws://headless-shell:9222