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/core/config.py

@@ -40,6 +40,8 @@ class Settings(BaseSettings):
     # CORS - set FRONTEND_URL in production (e.g., https://patherly.up.railway.app)
     CORS_ORIGINS: list[str] = ["http://localhost:3000", "http://localhost:5173", "http://localhost:5174"]
     FRONTEND_URL: Optional[str] = None
+    # Allow all Railway PR environments (set to True in Railway env vars)
+    ALLOW_RAILWAY_ORIGINS: bool = False
 
     @property
     def allowed_origins(self) -> list[str]:
@@ -49,6 +51,15 @@ class Settings(BaseSettings):
             origins.append(self.FRONTEND_URL)
         return origins
 
+    def is_origin_allowed(self, origin: str) -> bool:
+        """Check if an origin is allowed, including Railway wildcard pattern."""
+        if origin in self.allowed_origins:
+            return True
+        # Allow any *.up.railway.app origin for PR environments
+        if self.ALLOW_RAILWAY_ORIGINS and origin.endswith(".up.railway.app"):
+            return True
+        return False
+
     class Config:
         env_file = ".env"
         case_sensitive = True