feat: initial custom n8n image with n8n-nodes-upload-post@0.1.42

- Dockerfile based on n8nio/n8n:2.15.1
- Pre-installs n8n-nodes-upload-post@0.1.42 as PVC bootstrap
- CI/CD with SHA tags matching polymarket-bot pattern
- README with version bump process and migration warnings

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

Dockerfile

@@ -0,0 +1,15 @@
+FROM n8nio/n8n:2.15.1
+
+# Pre-install community node as bootstrap.
+# NOTE: at runtime the PVC mounts over /home/node/.n8n, so n8n uses the
+# version from the PVC. This installation acts as a fallback if the PVC
+# is ever lost or a fresh volume is used.
+USER root
+
+RUN mkdir -p /home/node/.n8n/nodes && \
+    cd /home/node/.n8n/nodes && \
+    echo '{"name":"installed-nodes","private":true,"dependencies":{}}' > package.json && \
+    npm install n8n-nodes-upload-post@0.1.42 && \
+    chown -R node:node /home/node/.n8n
+
+USER node