fix: add .dockerignore to exclude node_modules from build context

Without .dockerignore, COPY . . was overwriting the npm ci install
with the server's stale node_modules, causing Rollup to pick up the
wrong @radix-ui/react-slot version.

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

.dockerignore

@@ -0,0 +1,13 @@
+node_modules
+dist
+.git
+.claude
+backend/venv
+backend/__pycache__
+backend/uploads
+*.md
+*.log
+.env
+.env.*
+!.env.production
+.DS_Store