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.py

@@ -1,12 +1,15 @@
 import uuid
 from datetime import datetime, timezone
-from typing import Optional, Any
+from typing import Optional, Any, TYPE_CHECKING
 from sqlalchemy import String, DateTime, ForeignKey, Boolean, Text
 import sqlalchemy as sa
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from app.core.database import Base
 
+if TYPE_CHECKING:
+    from app.models.session_share import SessionShare
+
 
 class Session(Base):
     __tablename__ = "sessions"
@@ -53,3 +56,4 @@ class Session(Base):
     tree: Mapped["Tree"] = relationship("Tree", back_populates="sessions")
     user: Mapped["User"] = relationship("User", back_populates="sessions")
     attachments: Mapped[list["Attachment"]] = relationship("Attachment", back_populates="session")
+    shares: Mapped[list["SessionShare"]] = relationship("SessionShare", back_populates="session", cascade="all, delete-orphan")