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.py

@@ -1,9 +1,25 @@
 from datetime import datetime
-from typing import Optional, Any
+from typing import Optional, Any, Literal
 from uuid import UUID
 from pydantic import BaseModel, Field, validator
 
 
+class CustomStepSchema(BaseModel):
+    """Enhanced custom step with source tracking.
+
+    Backward compatible: old sessions without new fields load with defaults.
+    """
+    type: str  # "decision" | "action" | "solution"
+    content: str
+    notes: Optional[str] = None
+
+    # Source tracking (new fields, optional for backward compatibility)
+    source: Literal["ad-hoc", "step-library", "forked-tree"] = "ad-hoc"
+    source_step_id: Optional[UUID] = None
+    inserted_at: Optional[datetime] = None
+    inserted_after_node_id: Optional[str] = None
+
+
 class DecisionRecord(BaseModel):
     node_id: str
     question: Optional[str] = None
@@ -24,7 +40,7 @@ class SessionCreate(BaseModel):
 class SessionUpdate(BaseModel):
     path_taken: Optional[list[str]] = None
     decisions: Optional[list[DecisionRecord]] = None
-    custom_steps: Optional[list[dict[str, Any]]] = None
+    custom_steps: Optional[list[CustomStepSchema]] = None
     ticket_number: Optional[str] = Field(None, max_length=100)
     client_name: Optional[str] = Field(None, max_length=255)
     scratchpad: Optional[str] = None