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/Dockerfile

@@ -9,7 +9,11 @@ COPY go.mod go.sum ./
 RUN go mod download
 
 # Then source (changes often).
+# Source plus the go:embed'd assets. Missing either directory turns the embed
+# directive into a build error, so both must be copied before `go build`.
 COPY *.go ./
+COPY templates/ ./templates/
+COPY static/ ./static/
 
 # Static binary: pure-Go sqlite means CGO_ENABLED=0 -> no libc dependency.
 # -trimpath + -ldflags strip paths and debug info for a smaller image.