fix: add files to public directory for Docker build

Add placeholder files to ensure public directory is not empty:
- robots.txt for SEO
- .gitkeep to track directory in git

Fixes Docker buildx cache key calculation error where empty
public directory caused build to fail.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

public/.gitkeep

@@ -0,0 +1 @@
+# This file ensures the public directory is tracked by git

public/robots.txt

@@ -0,0 +1,3 @@
+# https://www.robotstxt.org/robotstxt.html
+User-agent: *
+Disallow: