feat: docker compose support

[me0106]

Summary

• Add Docker Compose setup for running memind-server and memind-ui together.
• Add a production-style memind-server Dockerfile that builds the Spring Boot jar in a Maven stage and runs it on
  JRE 21.
• Add a memind-ui Dockerfile using Caddy to serve the built Vite app.
• Add memind-ui/Caddyfile to serve SPA routes and reverse proxy /admin/* and /open/* to memind-server:8366.
• Add Docker ignore files to keep build contexts small.

How to Run

OPENAI_API_KEY=your-key docker compose up --build

Then open:

http://localhost:8080

The server is also exposed on:

http://localhost:8366

## Verification

- docker compose config --quiet
- docker compose build --check
- caddy validate --config /etc/caddy/Caddyfile
- docker compose up --build -d

## Notes

memind-server requires OPENAI_API_KEY at startup because Spring AI creates the embedding model during application
initialization. Without the key, the server container restarts with OpenAI API key must be set.