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>

.github/workflows/release.yml

@@ -0,0 +1,83 @@
+name: Release
+
+on:
+  push:
+    tags:
+      - "v*"
+
+permissions:
+  contents: write
+  packages: write
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+      - run: npm ci
+      - run: npm run typecheck
+      - run: npm test
+
+  docker:
+    runs-on: ubuntu-latest
+    needs: test
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/setup-buildx-action@v3
+
+      - uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract version from tag
+        id: version
+        run: echo "version=${GITHUB_REF#refs/tags/v}" >> "$GITHUB_OUTPUT"
+
+      - uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          tags: |
+            ghcr.io/${{ github.repository }}:${{ steps.version.outputs.version }}
+            ghcr.io/${{ github.repository }}:latest
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
+  github-release:
+    runs-on: ubuntu-latest
+    needs: test
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+          cache-dependency-path: extension/package-lock.json
+
+      - name: Build extensions
+        working-directory: extension
+        run: |
+          npm ci
+          for browser in chrome firefox edge safari; do
+            node esbuild.config.mjs --browser=$browser
+          done
+
+      - name: Package extensions
+        run: |
+          cd extension/build
+          for browser in chrome firefox edge safari; do
+            zip -r "../../cookiebridge-${browser}-${{ github.ref_name }}.zip" "$browser/"
+          done
+
+      - uses: softprops/action-gh-release@v2
+        with:
+          generate_release_notes: true
+          files: cookiebridge-*-${{ github.ref_name }}.zip