build: add Docker Compose setup for Dokploy deployment

- Multi-stage Dockerfile for Go backend (golang:1.25-alpine -> alpine:3)
- Multi-stage Dockerfile for Next.js frontend (bun:1 -> node:22-alpine)
- docker-compose.yml with backend + frontend services, health checks
- Next.js standalone output + API rewrites to proxy /api/* to backend
- .dockerignore files for both services
- .env.example documenting required environment variables

.env.example

@@ -0,0 +1,12 @@
+# KanzlAI Environment Variables
+# Copy to .env and fill in values: cp .env.example .env
+
+# Backend
+PORT=8080
+
+# Supabase (required for database access)
+SUPABASE_URL=
+SUPABASE_ANON_KEY=
+
+# Claude API (required for AI features)
+ANTHROPIC_API_KEY=