feat: replace StdioTransport with StreamableHTTP + Node.js cluster

Integrates StreamableHTTP natively into the MCP server instead of
using StdioServerTransport. Adds Node.js cluster support so multiple
workers can handle concurrent MCP clients simultaneously.

Changes:
- src/index.ts: replace StdioServerTransport with StreamableHTTPServerTransport
  + Node.js cluster (primary forks N workers, each worker handles HTTP
  requests on :8080/mcp with a serializing mutex)
- Dockerfile: multi-stage build (node:22-slim builder + runtime)
- .gitea/workflows/docker-build.yml: CI/CD pipeline — on push to main,
  builds and pushes Docker image to gitea.p-lao.com registry

Environment variables:
- MCP_WORKERS (default: 4) — number of cluster workers

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/docker-build.yml

@@ -0,0 +1,52 @@
+name: Docker Build & Push
+
+on:
+  push:
+    branches: [ main ]
+
+env:
+  REGISTRY: gitea.p-lao.com
+
+jobs:
+  build-and-push:
+    if: gitea.ref == 'refs/heads/main'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          github-server-url: https://gitea.p-lao.com
+          token: ${{ secrets.TOKEN_GITEA }}
+
+      - name: Get version
+        id: get_version
+        run: |
+          VERSION=$(node -p "require('./package.json').version")
+          if [ "$VERSION" != "" ]; then
+            echo "version=$VERSION" >> $GITEA_ENV
+          else
+            echo "Version not found in package.json"
+            exit 1
+          fi
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Gitea Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: |
+            gitea.p-lao.com/aleleba/docmost-mcp:${{ env.version }}
+            gitea.p-lao.com/aleleba/docmost-mcp:latest