feat: add Docker containerization (Dockerfile, compose, dockerignore)

Multi-stage build: frontend (Vite) + backend (TypeScript) in one container.
Production image based on node:20-alpine with health check.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,20 @@
+node_modules
+dist
+.git
+.gitignore
+.DS_Store
+*.md
+*.docx
+*.html
+.vscode
+.claude
+.mcp.json
+tools.yaml
+backend/public
+backend/node_modules
+backend/dist
+backend/.env
+frontend/node_modules
+frontend/dist
+prototyp_*.html
+projektplan_*