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_library.py

@@ -0,0 +1,135 @@
+from datetime import datetime
+from decimal import Decimal
+from typing import Optional, Literal
+from uuid import UUID
+from pydantic import BaseModel, Field
+
+
+class StepCommand(BaseModel):
+    """A command that can be run as part of a step."""
+    label: str
+    command: str
+    command_type: Optional[str] = None  # e.g., 'powershell', 'cmd', 'bash'
+
+
+class StepContent(BaseModel):
+    """Content structure for step library entries (stored as JSONB)."""
+    instructions: str = Field(..., min_length=1)
+    help_text: Optional[str] = None
+    commands: Optional[list[StepCommand]] = None
+
+
+# Base schemas
+class StepLibraryBase(BaseModel):
+    title: str = Field(..., min_length=1, max_length=255)
+    step_type: Literal['decision', 'action', 'solution']
+    content: StepContent
+    category_id: Optional[UUID] = None
+    tags: list[str] = Field(default_factory=list)
+    visibility: Literal['private', 'team', 'public'] = 'private'
+
+
+class StepLibraryCreate(StepLibraryBase):
+    team_id: Optional[UUID] = None
+
+
+class StepLibraryUpdate(BaseModel):
+    title: Optional[str] = Field(None, min_length=1, max_length=255)
+    step_type: Optional[Literal['decision', 'action', 'solution']] = None
+    content: Optional[StepContent] = None
+    category_id: Optional[UUID] = None
+    tags: Optional[list[str]] = None
+    visibility: Optional[Literal['private', 'team', 'public']] = None
+
+
+class StepLibraryResponse(StepLibraryBase):
+    id: UUID
+    created_by: UUID
+    team_id: Optional[UUID] = None
+    usage_count: int
+    rating_average: Decimal
+    rating_count: int
+    helpful_yes: int
+    helpful_no: int
+    is_featured: bool
+    is_verified: bool
+    is_active: bool
+    created_at: datetime
+    updated_at: datetime
+    # Computed fields (populated by API)
+    category_name: Optional[str] = None
+    author_name: Optional[str] = None
+
+    class Config:
+        from_attributes = True
+
+
+class StepLibraryListResponse(BaseModel):
+    id: UUID
+    title: str
+    step_type: str
+    visibility: str
+    category_id: Optional[UUID] = None
+    category_name: Optional[str] = None
+    tags: list[str]
+    usage_count: int
+    rating_average: Decimal
+    rating_count: int
+    is_featured: bool
+    created_by: UUID
+    author_name: Optional[str] = None
+    created_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+# Rating schemas
+class StepRatingBase(BaseModel):
+    rating: int = Field(..., ge=1, le=5)
+    was_helpful: Optional[bool] = None
+    review_text: Optional[str] = Field(None, max_length=500)
+
+
+class StepRatingCreate(StepRatingBase):
+    session_id: Optional[UUID] = None  # For verified use tracking
+
+
+class StepRatingUpdate(BaseModel):
+    rating: Optional[int] = Field(None, ge=1, le=5)
+    was_helpful: Optional[bool] = None
+    review_text: Optional[str] = Field(None, max_length=500)
+
+
+class StepRatingResponse(StepRatingBase):
+    id: UUID
+    step_id: UUID
+    user_id: UUID
+    is_verified_use: bool
+    session_id: Optional[UUID] = None
+    is_visible: bool
+    created_at: datetime
+    updated_at: datetime
+    # Computed
+    user_name: Optional[str] = None
+
+    class Config:
+        from_attributes = True
+
+
+# Search and filter schemas
+class StepSearchParams(BaseModel):
+    q: Optional[str] = None  # Full-text search query
+    category_id: Optional[UUID] = None
+    tags: Optional[list[str]] = None
+    min_rating: Optional[float] = Field(None, ge=0, le=5)
+    step_type: Optional[Literal['decision', 'action', 'solution']] = None
+    visibility: Optional[Literal['private', 'team', 'public']] = None
+    sort_by: Literal['recent', 'popular', 'highest_rated', 'most_used'] = 'recent'
+    limit: int = Field(20, ge=1, le=100)
+    offset: int = Field(0, ge=0)
+
+
+class PopularTagResponse(BaseModel):
+    tag: str
+    count: int