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/pages/SessionDetailPage.tsx

@@ -3,17 +3,19 @@ import { useParams, useNavigate } from 'react-router-dom'
 import { Copy, Check, Eye } from 'lucide-react'
 import { sessionsApi } from '@/api'
 import { ExportPreviewModal } from '@/components/session/ExportPreviewModal'
+import { useUserPreferencesStore } from '@/store/userPreferencesStore'
 import type { Session, SessionExport } from '@/types'
 import { cn } from '@/lib/utils'
 
 export function SessionDetailPage() {
   const { id } = useParams<{ id: string }>()
   const navigate = useNavigate()
+  const { defaultExportFormat } = useUserPreferencesStore()
   const [session, setSession] = useState<Session | null>(null)
   const [isLoading, setIsLoading] = useState(true)
   const [error, setError] = useState<string | null>(null)
   const [isExporting, setIsExporting] = useState(false)
-  const [exportFormat, setExportFormat] = useState<'markdown' | 'text' | 'html'>('markdown')
+  const [exportFormat, setExportFormat] = useState<'markdown' | 'text' | 'html'>(defaultExportFormat)
   const [exportContent, setExportContent] = useState<string | null>(null)
   const [showPreview, setShowPreview] = useState(false)
   const [copied, setCopied] = useState(false)