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/tree.py

@@ -40,6 +40,24 @@ class TreeUpdate(BaseModel):
     tags: Optional[list[str]] = Field(None, max_length=10, description="List of tag names to assign (replaces existing)")
 
 
+class ForkCreate(BaseModel):
+    fork_reason: Optional[str] = Field(None, max_length=255, description="Brief reason for forking")
+    name: Optional[str] = Field(None, min_length=1, max_length=255, description="Name for the fork (defaults to 'Fork of {original name}')")
+
+
+class ForkInfo(BaseModel):
+    """Fork metadata included in tree responses."""
+    parent_tree_id: Optional[UUID] = None
+    root_tree_id: Optional[UUID] = None
+    fork_reason: Optional[str] = None
+    fork_depth: int = 0
+    parent_updated_at: Optional[datetime] = None
+    has_parent_updates: bool = False
+
+    class Config:
+        from_attributes = True
+
+
 class TreeResponse(TreeBase):
     id: UUID
     tree_structure: dict[str, Any]
@@ -48,6 +66,7 @@ class TreeResponse(TreeBase):
     category_id: Optional[UUID] = None
     category_info: Optional[CategoryInfo] = None
     tags: list[str] = []  # List of tag names
+    fork_info: Optional[ForkInfo] = None
     is_active: bool
     is_public: bool
     is_default: bool