chore: build, route, and document the web UI

Fix backend/Dockerfile to COPY templates/ and static/ (the go:embed
assets from Tasks 5-7) alongside *.go, plus backend/.dockerignore which
was silently excluding both directories from the build context — the
Dockerfile fix alone still failed the build. Wire WEB_PASSWORD through
docker-compose.yml, add a second Traefik router (mangaweb) plus explicit
service labels on both routers in docker-compose.prod.yml, and document
the new variables and deploy steps in .env.example, DEPLOY.md, and
CLAUDE.md.

docker-compose.yml

@@ -18,6 +18,8 @@ services:
       ALLOWED_ORIGINS: ${ALLOWED_ORIGINS:-https://asuracomic.net,https://asurascans.com,https://demonicscans.org}
       DB_PATH: /data/bookmarks.db
       PORT: "8080"
+      # Gates the browser UI. Unset means the web routes are not served at all.
+      WEB_PASSWORD: ${WEB_PASSWORD:-}
     volumes:
       - bookmarks-data:/data
     # Bound to loopback only: the proxy (or curl during smoke test) reaches it,