ci: add Gitea Actions CI/CD with Portainer deployment

Add complete CI/CD pipeline:
- Gitea Actions workflow (build + deploy)
- Multi-stage Dockerfile (Node build + nginx serve)
- nginx config with SPA routing and cache headers
- docker-compose.yml for local testing
- .dockerignore to optimize build

Pipeline flow:
1. Build job: npm ci + npm build + upload artifact
2. Deploy job (main only): Docker build + push to registry + Portainer webhook

Requires Gitea secrets:
- DOCKER_USERNAME
- DOCKER_PASSWORD
- PORTAINER_WEBHOOK_URL

Dockerfile

@@ -0,0 +1,36 @@
+# Multi-stage build for Astro static site
+
+# Stage 1: Build
+FROM node:20-alpine AS builder
+
+WORKDIR /app
+
+# Copy package files
+COPY package*.json ./
+
+# Install dependencies
+RUN npm ci --only=production
+
+# Copy source
+COPY . .
+
+# Build static site
+RUN npm run build
+
+# Stage 2: Production - nginx
+FROM nginx:alpine
+
+# Copy built site to nginx html directory
+COPY --from=builder /app/dist /usr/share/nginx/html
+
+# Copy nginx config (if needed for SPA routing)
+COPY nginx.conf /etc/nginx/nginx.conf
+
+# Expose port
+EXPOSE 80
+
+# Health check
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD wget --no-verbose --tries=1 --spider http://localhost/ || exit 1
+
+CMD ["nginx", "-g", "daemon off;"]