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.

backend/.dockerignore

@@ -1,5 +1,10 @@
-# Only go source + module files are needed in the build context.
+# Only go source + module files, plus the go:embed'd templates/static
+# directories, are needed in the build context.
 *
 !go.mod
 !go.sum
 !*.go
+!templates/
+!templates/**
+!static/
+!static/**