feat: Phase 1 Group 10 — create global content tables and platform account

Creates template_trees and platform_steps (no account_id, no RLS).
Migrates is_default=TRUE trees and public steps into them.
Creates sentinel platform account (00000000-...-0001) for global
tree_categories, tree_tags, step_categories, step_library, and
is_default trees — clearing all NULL account_id rows in those tables
as prerequisite for Group 9 SET NOT NULL.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -54,6 +54,8 @@ from .session_branch import SessionBranch
 from .fork_point import ForkPoint
 from .session_handoff import SessionHandoff
 from .session_resolution_output import SessionResolutionOutput
+from .template_tree import TemplateTree
+from .platform_step import PlatformStep
 
 __all__ = [
     "User",
@@ -122,4 +124,6 @@ __all__ = [
     "ForkPoint",
     "SessionHandoff",
     "SessionResolutionOutput",
+    "TemplateTree",
+    "PlatformStep",
 ]