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/tree.py

@@ -79,10 +79,62 @@ class Tree(Base):
     )
     usage_count: Mapped[int] = mapped_column(Integer, default=0)
 
+    # Fork tracking
+    parent_tree_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("trees.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True
+    )
+    fork_reason: Mapped[Optional[str]] = mapped_column(
+        String(255),
+        nullable=True,
+        comment="Brief reason: 'Added Cisco Meraki steps for our network'"
+    )
+    parent_updated_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True,
+        comment="Snapshot of parent's updated_at when fork created. Compare to detect parent updates."
+    )
+
+    # Fork lineage tracking
+    root_tree_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("trees.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+        comment="Original tree at root of fork chain (NULL for non-forked trees)"
+    )
+    fork_depth: Mapped[int] = mapped_column(
+        Integer,
+        nullable=False,
+        default=0,
+        server_default="0",
+        comment="Fork depth: 0 = original, 1 = direct fork, 2 = fork of fork, etc."
+    )
+
     # Relationships
     author: Mapped[Optional["User"]] = relationship("User", foreign_keys=[author_id], back_populates="trees")
     team: Mapped[Optional["Team"]] = relationship("Team", back_populates="trees")
     account: Mapped[Optional["Account"]] = relationship("Account", foreign_keys=[account_id], back_populates="trees")
+
+    # Fork relationships (self-referential)
+    parent: Mapped[Optional["Tree"]] = relationship(
+        "Tree",
+        remote_side="Tree.id",
+        foreign_keys=[parent_tree_id],
+        back_populates="forks"
+    )
+    forks: Mapped[list["Tree"]] = relationship(
+        "Tree",
+        foreign_keys=[parent_tree_id],
+        back_populates="parent"
+    )
+    root: Mapped[Optional["Tree"]] = relationship(
+        "Tree",
+        remote_side="Tree.id",
+        foreign_keys=[root_tree_id]
+    )
     sessions: Mapped[list["Session"]] = relationship("Session", back_populates="tree")
 
     # New organization relationships