feat(06-01): add Docker configuration for production deployment

- Create Dockerfile with multi-stage build using node:22-alpine
- Run as non-root 'nodejs' user for security
- Add HEALTHCHECK for container orchestration
- Create docker-compose.yml with taskplaner_data named volume
- Add .dockerignore to optimize build context

.dockerignore

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+node_modules
+build
+.svelte-kit
+data
+.git
+.gitignore
+.env*
+*.md
+.planning
+.vscode

Dockerfile

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+# Build stage
+FROM node:22-alpine AS builder
+
+WORKDIR /app
+
+# Copy package files
+COPY package*.json ./
+
+# Install dependencies
+RUN npm ci
+
+# Copy source code
+COPY . .
+
+# Build the application
+RUN npm run build
+
+# Prune dev dependencies
+RUN npm prune --production
+
+# Production stage
+FROM node:22-alpine AS production
+
+WORKDIR /app
+
+# Create non-root user
+RUN addgroup -g 1001 -S nodejs && \
+    adduser -S nodejs -u 1001
+
+# Copy built application from builder
+COPY --from=builder --chown=nodejs:nodejs /app/build ./build
+COPY --from=builder --chown=nodejs:nodejs /app/node_modules ./node_modules
+COPY --from=builder --chown=nodejs:nodejs /app/package.json ./
+
+# Create data directory with proper ownership
+RUN mkdir -p /app/data && chown -R nodejs:nodejs /app/data
+
+# Switch to non-root user
+USER nodejs
+
+# Set environment variables
+ENV NODE_ENV=production
+ENV TASKPLANER_DATA_DIR=/app/data
+ENV PORT=3000
+
+# Expose port
+EXPOSE 3000
+
+# Health check (endpoint created in Plan 02)
+HEALTHCHECK --interval=30s --timeout=10s --start-period=5s --retries=3 \
+  CMD wget --no-verbose --tries=1 --spider http://localhost:3000/health || exit 1
+
+# Start the application
+CMD ["node", "build/index.js"]

docker-compose.yml

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+services:
+  taskplaner:
+    build: .
+    container_name: taskplaner
+    ports:
+      - "3000:3000"
+    volumes:
+      - taskplaner_data:/app/data
+    environment:
+      - NODE_ENV=production
+      - TASKPLANER_DATA_DIR=/app/data
+    restart: unless-stopped
+
+volumes:
+  taskplaner_data: