fix: use slug from frontmatter if present in document migration

- Previously the migration script always generated slugs from titles
- Now checks metadata.slug first before auto-generating
- Allows explicit slug control via frontmatter (e.g., glossary-de, glossary-fr)
- Required for language-specific document API fallback to work correctly

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

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

scripts/migrate-documents.js

@@ -140,8 +140,8 @@ async function processMarkdownFile(filePath, sourcePath) {
   // Extract table of contents
   const tableOfContents = extractTOC(content);
 
-  // Generate slug from title
-  const slug = generateSlug(metadata.title);
+  // Use slug from frontmatter if present, otherwise generate from title
+  const slug = metadata.slug || generateSlug(metadata.title);
 
   // Determine if document should be public
   // Internal document patterns (should NOT be public)