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/schemas/step_category.py

@@ -0,0 +1,58 @@
+from datetime import datetime
+from typing import Optional
+from uuid import UUID
+from pydantic import BaseModel, Field
+import re
+
+
+def slugify(name: str) -> str:
+    """Convert a name to a URL-safe slug."""
+    # Remove non-alphanumeric chars except spaces, convert to lowercase
+    slug = re.sub(r'[^a-zA-Z0-9 ]', '', name.lower())
+    # Replace spaces with hyphens
+    slug = re.sub(r' +', '-', slug.strip())
+    return slug
+
+
+class StepCategoryBase(BaseModel):
+    name: str = Field(..., min_length=1, max_length=100)
+    description: Optional[str] = None
+
+
+class StepCategoryCreate(StepCategoryBase):
+    team_id: Optional[UUID] = Field(None, description="Team ID for team-specific category. NULL for global.")
+
+
+class StepCategoryUpdate(BaseModel):
+    name: Optional[str] = Field(None, min_length=1, max_length=100)
+    description: Optional[str] = None
+    display_order: Optional[int] = None
+    is_active: Optional[bool] = None
+
+
+class StepCategoryResponse(StepCategoryBase):
+    id: UUID
+    slug: str
+    team_id: Optional[UUID] = None
+    display_order: int
+    is_active: bool
+    created_at: datetime
+    updated_at: datetime
+    step_count: int = 0  # Computed field - count of steps in this category
+
+    class Config:
+        from_attributes = True
+
+
+class StepCategoryListResponse(BaseModel):
+    id: UUID
+    name: str
+    slug: str
+    description: Optional[str] = None
+    team_id: Optional[UUID] = None
+    display_order: int
+    is_active: bool
+    step_count: int = 0
+
+    class Config:
+        from_attributes = True