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/store/userPreferencesStore.ts

@@ -0,0 +1,23 @@
+import { create } from 'zustand'
+import { persist } from 'zustand/middleware'
+
+type ExportFormat = 'markdown' | 'text' | 'html'
+
+interface UserPreferencesState {
+  defaultExportFormat: ExportFormat
+  setDefaultExportFormat: (format: ExportFormat) => void
+}
+
+export const useUserPreferencesStore = create<UserPreferencesState>()(
+  persist(
+    (set) => ({
+      defaultExportFormat: 'markdown',
+      setDefaultExportFormat: (format) => set({ defaultExportFormat: format }),
+    }),
+    {
+      name: 'user-preferences-storage',
+    }
+  )
+)
+
+export default useUserPreferencesStore