feat: add tree forking, custom step tracking, and session sharing

Implement three foundational schema features from the design doc:

- Tree forking with lineage tracking (migration 022): parent_tree_id,
  root_tree_id, fork_depth columns with self-referential FKs and
  composite analytics index
- Custom step enhancement: CustomStepSchema with source tracking
  (ad-hoc, step-library, forked-tree) for backward-compatible JSONB
- Session sharing (migration 023): session_shares and session_share_views
  tables with account-scoped visibility, cryptographic tokens, view
  tracking, and allow_public_shares account policy

Includes 21 new integration tests (9 forking, 12 sharing), SaaS
consultant-recommended denormalizations, rate limiting on public share
access, and test fixture fix for invite code requirement.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/session_share.py

@@ -0,0 +1,152 @@
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, TYPE_CHECKING
+from sqlalchemy import String, DateTime, ForeignKey, Boolean, Integer, CheckConstraint
+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.session import Session
+    from app.models.user import User
+    from app.models.account import Account
+
+
+class SessionShare(Base):
+    __tablename__ = "session_shares"
+    __table_args__ = (
+        CheckConstraint(
+            "visibility IN ('public', 'account')",
+            name='ck_session_shares_visibility'
+        ),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        primary_key=True,
+        default=uuid.uuid4
+    )
+    session_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("sessions.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+        comment="Account that owns this share (denormalized from session at creation)"
+    )
+    share_token: Mapped[str] = mapped_column(
+        String(64),
+        unique=True,
+        nullable=False,
+        index=True,
+        comment="URL-safe random token (48 bytes -> 64 base64 chars)"
+    )
+    share_name: Mapped[Optional[str]] = mapped_column(
+        String(100),
+        nullable=True,
+        comment="Optional label: 'Training link', 'Customer escalation #1234'"
+    )
+    visibility: Mapped[str] = mapped_column(
+        String(20),
+        nullable=False,
+        default="public",
+        comment="public = anyone with link, account = account members only"
+    )
+    created_by: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+        index=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)
+    )
+    expires_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True,
+        index=True,
+        comment="Optional expiration for time-limited shares"
+    )
+    view_count: Mapped[int] = mapped_column(
+        Integer,
+        nullable=False,
+        default=0
+    )
+    last_viewed_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True
+    )
+    is_active: Mapped[bool] = mapped_column(
+        Boolean,
+        nullable=False,
+        default=True,
+        index=True
+    )
+
+    # Relationships
+    session: Mapped["Session"] = relationship("Session", back_populates="shares")
+    account: Mapped["Account"] = relationship("Account")
+    creator: Mapped["User"] = relationship("User", foreign_keys=[created_by])
+    views: Mapped[list["SessionShareView"]] = relationship(
+        "SessionShareView",
+        back_populates="share",
+        cascade="all, delete-orphan"
+    )
+
+
+class SessionShareView(Base):
+    __tablename__ = "session_share_views"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        primary_key=True,
+        default=uuid.uuid4
+    )
+    share_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("session_shares.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+    session_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("sessions.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+        comment="Denormalized from share for analytics queries"
+    )
+    viewer_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+        comment="NULL for public shares (unauthenticated views)"
+    )
+    viewed_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        index=True
+    )
+    viewer_ip: Mapped[Optional[str]] = mapped_column(
+        String(45),  # IPv6 max length
+        nullable=True
+    )
+    viewer_user_agent: Mapped[Optional[str]] = mapped_column(
+        String(500),
+        nullable=True
+    )
+
+    # Relationships
+    share: Mapped["SessionShare"] = relationship("SessionShare", back_populates="views")
+    viewer: Mapped[Optional["User"]] = relationship("User")