feat: deploy (Dockerfile, compose) and Gitea Actions CI

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/Dockerfile

@@ -0,0 +1,14 @@
+# Build context is the repository root:
+#   docker build -f deploy/Dockerfile --build-arg COURSE_HOST=course.example.com .
+FROM node:22-alpine AS build
+ARG COURSE_HOST
+RUN test -n "$COURSE_HOST" || (echo "COURSE_HOST build arg is required" >&2; exit 1)
+ENV COURSE_HOST=$COURSE_HOST
+WORKDIR /app
+COPY . ./
+RUN npm --prefix site ci --no-audit --no-fund
+RUN npm --prefix site run build
+
+FROM nginx:alpine
+COPY --from=build /app/site/dist /usr/share/nginx/html
+EXPOSE 80