docs: add legacy notice to monolithic API reference

- Add warning banner to api-reference.md (1,600+ lines)
- Point users to new split endpoint documentation
- Recommend API Documentation Portal for interactive explorer
- Clarify this is kept for backward compatibility
- Resolves device content warnings (appropriate as legacy reference)

docs/developer/api-reference.md

@@ -1,5 +1,16 @@
 # Bookhoard API Reference
 
+> **⚠️ Legacy Document**: This is a monolithic API reference (1,600+ lines).
+> For updated, split endpoint documentation with interactive API explorer, see [API Documentation Portal](api/INDEX.md).
+>
+> **Use the split docs for:**
+> - Easier navigation by category
+> - Interactive API explorer
+> - Endpoint-specific examples
+> - Latest updates
+
+---
+
 Complete API documentation for Bookhoard v1.0 with Universal Cross-Platform Sync support.
 
 ## Table of Contents