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/step_category.py

@@ -0,0 +1,64 @@
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, TYPE_CHECKING
+from sqlalchemy import String, Text, DateTime, ForeignKey, Boolean, Integer, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+from app.core.database import Base
+
+if TYPE_CHECKING:
+    from app.models.team import Team
+    from app.models.user import User
+
+
+class StepCategory(Base):
+    """Admin-managed categories for organizing step library entries.
+
+    Categories can be:
+    - Global (team_id=NULL): Created by Patherly admins, visible to all
+    - Team-specific (team_id set): Created by team admins, visible to team members
+    """
+    __tablename__ = "step_categories"
+    __table_args__ = (
+        UniqueConstraint('slug', 'team_id', name='uq_step_categories_slug_team'),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        primary_key=True,
+        default=uuid.uuid4
+    )
+    name: Mapped[str] = mapped_column(String(100), nullable=False)
+    slug: Mapped[str] = mapped_column(String(100), nullable=False, index=True)
+    description: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    team_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("teams.id", ondelete="CASCADE"),
+        nullable=True,
+        index=True
+    )
+    display_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0, index=True)
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    created_by: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="SET NULL"),
+        nullable=True
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc)
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc)
+    )
+
+    # Relationships
+    team: Mapped[Optional["Team"]] = relationship("Team", back_populates="step_categories")
+    creator: Mapped[Optional["User"]] = relationship("User", foreign_keys=[created_by])
+
+    @property
+    def is_global(self) -> bool:
+        """Returns True if this is a global category (not team-specific)."""
+        return self.team_id is None