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>

docker-compose.yml

@@ -0,0 +1,15 @@
+services:
+  relay:
+    build: .
+    ports:
+      - "${PORT:-8080}:8080"
+    environment:
+      - PORT=8080
+      - HOST=0.0.0.0
+    restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "wget", "-qO-", "http://localhost:8080/health"]
+      interval: 30s
+      timeout: 5s
+      retries: 3
+      start_period: 10s