feat: implement tree sharing, draft trees, and session-to-tree conversion (Issues #16, #25, #17)

Backend features:
- Tree sharing via secure tokens with expiration (Issue #16)
- Draft tree status with conditional validation (Issue #25)
- Save session as custom tree with fork tracking (Issue #17)
- Tree validation system for publish requirements
- Session-to-tree conversion preserving custom steps

Database migrations:
- 024: Tree sharing (tree_shares table, visibility field)
- 025: Tree status field (draft/published)
- 25b: Merge migration for indexes

New endpoints:
- POST /api/v1/trees/{id}/share - Generate share token
- GET /api/v1/shared/{token} - Public tree access
- POST /api/v1/trees/{id}/can-publish - Validate tree
- POST /api/v1/sessions/{id}/save-as-tree - Convert session

Test coverage:
- 20 tests for draft trees functionality
- 14 tests for session-to-tree conversion
- 15 tests for tree sharing

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/schemas/session.py

@@ -77,3 +77,17 @@ class SessionExport(BaseModel):
 
 class ScratchpadUpdate(BaseModel):
     scratchpad: str
+
+
+class SaveAsTreeRequest(BaseModel):
+    """Request to save a session as a tree."""
+    tree_name: Optional[str] = Field(None, max_length=255, description="Custom name for the saved tree (auto-generated if not provided)")
+    description: Optional[str] = Field(None, description="Description for the saved tree")
+    status: Literal["draft", "published"] = Field("draft", description="Status of the saved tree")
+
+
+class SaveAsTreeResponse(BaseModel):
+    """Response after saving a session as a tree."""
+    tree_id: UUID
+    tree_name: str
+    message: str