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/schemas/session_share.py

@@ -0,0 +1,47 @@
+from datetime import datetime
+from typing import Optional, Literal
+from uuid import UUID
+from pydantic import BaseModel, Field
+
+
+class ShareCreate(BaseModel):
+    visibility: Literal["public", "account"] = Field("public", description="Share visibility")
+    share_name: Optional[str] = Field(None, max_length=100, description="Optional label for the share")
+    expires_at: Optional[datetime] = Field(None, description="Optional expiration datetime")
+
+
+class ShareResponse(BaseModel):
+    id: UUID
+    session_id: UUID
+    account_id: UUID
+    share_token: str
+    share_name: Optional[str] = None
+    visibility: str
+    created_by: UUID
+    created_at: datetime
+    updated_at: datetime
+    expires_at: Optional[datetime] = None
+    view_count: int
+    last_viewed_at: Optional[datetime] = None
+    is_active: bool
+    share_url: Optional[str] = None
+
+    class Config:
+        from_attributes = True
+
+
+class SharePublicView(BaseModel):
+    """Read-only session data returned when accessing a share link."""
+    session_id: UUID
+    tree_name: str
+    tree_description: Optional[str] = None
+    tree_structure: dict
+    path_taken: list[str]
+    decisions: list[dict]
+    custom_steps: list[dict] = Field(default_factory=list)
+    started_at: datetime
+    completed_at: Optional[datetime] = None
+    ticket_number: Optional[str] = None
+    client_name: Optional[str] = None
+    share_name: Optional[str] = None
+    visibility: str