feat: Docker-Support (Compose + Dockerfiles)

- docker-compose.yml: db (timescaledb/pg17) + backend + frontend
- backend/Dockerfile: multi-stage (golang:1.24-alpine -> debian:bookworm-slim)
- frontend/Dockerfile: multi-stage (node:22-alpine -> nginx:alpine), Build-Args fuer VITE_*
- frontend/nginx.conf: SPA-Routing + Asset-Caching
- .env.example: BACKEND_HOST, DB_PASSWORD, JWT_SECRET, API_KEY
- backend/config/config.go: ENV-Var-Support vervollstaendigt (JWT_SECRET, API_KEY, DB_PORT, DB_USER, DB_NAME)
- backend/config.yaml.example: ENV-Var Kommentare ergaenzt
- README: Docker-Quickstart als empfohlener Einstieg (3 Befehle)
- .gitignore: .env zur ignore-Liste hinzugefuegt

frontend/Dockerfile

@@ -0,0 +1,29 @@
+FROM node:22-alpine AS builder
+
+WORKDIR /app
+COPY package*.json ./
+RUN npm ci
+
+COPY . .
+
+# Backend-URL wird zur Build-Zeit via ARG gesetzt
+ARG VITE_BACKEND_HOST=localhost
+ARG VITE_BACKEND_PORT=8443
+ARG VITE_API_KEY=""
+
+ENV VITE_BACKEND_HOST=$VITE_BACKEND_HOST \
+    VITE_BACKEND_PORT=$VITE_BACKEND_PORT \
+    VITE_API_KEY=$VITE_API_KEY
+
+RUN npm run build
+
+# ---
+
+FROM nginx:alpine
+
+COPY --from=builder /app/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+
+EXPOSE 80
+
+CMD ["nginx", "-g", "daemon off;"]