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>

backend/Dockerfile

@@ -1,7 +1,7 @@
 # syntax=docker/dockerfile:1
 
 # --- build stage: compile a static, CGO-free binary ---
-FROM golang:1.24-alpine AS build
+FROM golang:1.26-alpine AS build
 WORKDIR /src
 
 # Dependencies first for layer caching (changes rarely).

backend/main.go

@@ -274,5 +274,20 @@ func startLatestPoller(ctx context.Context, s *store.Store, cfg LatestPoll) {
 		Stagger:  cfg.Stagger,
 		Batch:    cfg.Batch,
 	}
+
+	// Optional: without it, sites behind a JavaScript challenge are simply not
+	// polled, and their latest_chapter comes from the userscript alone — which
+	// is how the service behaved before the sidecar existed.
+	if ws := strings.TrimSpace(os.Getenv("BROWSER_WS_URL")); ws != "" {
+		bf, err := latest.NewBrowserFetcher(ws)
+		if err != nil {
+			log.Printf("latest-chapter poller: browser fetcher disabled: %v", err)
+		} else {
+			p.BrowserFetch = bf
+			context.AfterFunc(ctx, bf.Close)
+			log.Printf("latest-chapter poller: browser fetcher at %s", ws)
+		}
+	}
+
 	go p.Run(ctx)
 }