Initial monorepo scaffold

Set up top-level structure (backend, frontend, database, infra, shared,
docs) with .gitkeep placeholders, root README, and .gitignore.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,37 @@
+# oh-my-claudecode local state
+.omc/
+
+# Dependencies
+node_modules/
+.pnp
+.pnp.js
+
+# Build output
+dist/
+build/
+out/
+*.tsbuildinfo
+
+# Environment / secrets
+.env
+.env.*
+!.env.example
+
+# Logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+
+# OS / editor
+.DS_Store
+Thumbs.db
+.idea/
+.vscode/
+*.swp
+
+# Coverage / caches
+coverage/
+.cache/
+.turbo/