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

backend/app/models/__init__.py

@@ -7,6 +7,8 @@ from .invite_code import InviteCode
 from .category import TreeCategory
 from .tag import TreeTag, tree_tag_assignments
 from .folder import UserFolder, user_folder_trees
+from .step_category import StepCategory
+from .step_library import StepLibrary, StepRating, StepUsageLog
 
 __all__ = [
     "User",
@@ -20,4 +22,8 @@ __all__ = [
     "tree_tag_assignments",
     "UserFolder",
     "user_folder_trees",
+    "StepCategory",
+    "StepLibrary",
+    "StepRating",
+    "StepUsageLog",
 ]