feat: implement M4 self-hosting, Docker & documentation

- Dockerfile with multi-stage build (Node 22 Alpine, sodium-native)
- docker-compose.yml with health check for easy self-hosting
- README with setup guide, API reference, and project overview
- Architecture docs (data flow, component breakdown, protocol constants)
- Security model docs (threat model, crypto primitives, self-hosting checklist)
- GitHub Actions CI pipeline (test, typecheck, Docker smoke test, extension builds)
- GitHub Actions release pipeline (GHCR push, extension zip artifacts)
- CONTRIBUTING.md with dev setup and code style guidelines

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.dockerignore

@@ -0,0 +1,11 @@
+node_modules
+dist
+extension
+tests
+.git
+.github
+docs
+*.md
+!package.json
+!package-lock.json
+.DS_Store