feat(PROJ-27): Docker-Support + install.sh v2.0 (native + Docker-Modus)

Adds multi-stage Dockerfiles (Go+Xapian CGO, Next.js standalone),
docker-compose.yml, webhook-basierter Deploy-Flow und erweitertes
install.sh mit interaktiver Modus-Auswahl.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile.web

@@ -0,0 +1,31 @@
+# ── Stage 1: Next.js Build ────────────────────────────────────────────────────
+FROM node:22-bookworm-slim AS node-builder
+
+WORKDIR /build
+COPY package.json package-lock.json ./
+RUN npm ci
+
+COPY . .
+RUN npm run build
+
+# ── Stage 2: Runtime (Next.js standalone) ─────────────────────────────────────
+FROM node:22-bookworm-slim
+
+WORKDIR /app
+
+RUN useradd --system --shell /bin/false --uid 998 --create-home nextjs
+
+COPY --from=node-builder /build/.next/standalone ./
+COPY --from=node-builder /build/.next/static ./.next/static
+COPY --from=node-builder /build/public ./public
+
+RUN chown -R nextjs:nextjs /app
+
+USER nextjs
+EXPOSE 3000
+
+ENV PORT=3000
+ENV HOSTNAME=0.0.0.0
+# NEXT_PUBLIC_API_URL wird zur Laufzeit gesetzt (docker-compose environment)
+
+CMD ["node", "server.js"]