Add step library foundation and user preferences (#24)

## Summary
Implements Phase 2.5 Step Library Foundation:

### Issues Completed
- #3 User Preferences - export format default setting
- #5 Step Categories - database table and seed data  
- #6 Step Library - database schema and migrations
- #7 Step Library - CRUD API endpoints
- #8 Step Library - rating and review system

### Changes
**Backend:**
- Migration 007: step_categories table with 10 seeded global categories
- Migration 008: step_library, step_ratings, step_usage_log tables
- Full CRUD API for step categories (/api/v1/step-categories)
- Full CRUD API for step library (/api/v1/steps) with search, filters, ratings
- CORS support for Railway PR environments (ALLOW_RAILWAY_ORIGINS)

**Frontend:**
- User preferences store (Zustand + localStorage)
- Settings page at /settings with export format dropdown
- Default export format applied in SessionDetailPage

### Testing
- Tested in Railway PR environment
- Database seeded with 7 MSP troubleshooting trees
- All API endpoints verified working

frontend/src/components/layout/AppLayout.tsx

@@ -16,6 +16,7 @@ export function AppLayout() {
   const navItems = [
     { path: '/trees', label: 'Trees' },
     { path: '/sessions', label: 'Sessions' },
+    { path: '/settings', label: 'Settings' },
   ]
 
   return (