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/tests/test_phase1_migrations.py

@@ -478,3 +478,31 @@ async def test_target_list_account_id_from_team_admin(test_db: AsyncSession):
     )
     row = result.scalar_one()
     assert row.account_id == account.id
+
+
+# ── Group 10 (runs first): Global content tables ──────────────────────────────
+
+@pytest.mark.asyncio
+async def test_template_trees_table_exists_and_has_no_account_id(test_db: AsyncSession):
+    """template_trees must exist and must NOT have an account_id column."""
+    result = await test_db.execute(text("""
+        SELECT column_name
+        FROM information_schema.columns
+        WHERE table_name = 'template_trees'
+    """))
+    columns = {row[0] for row in result.fetchall()}
+    assert 'id' in columns, "template_trees.id must exist"
+    assert 'account_id' not in columns, "template_trees must not have account_id (global content)"
+
+
+@pytest.mark.asyncio
+async def test_platform_steps_table_exists_and_has_no_account_id(test_db: AsyncSession):
+    """platform_steps must exist and must NOT have an account_id column."""
+    result = await test_db.execute(text("""
+        SELECT column_name
+        FROM information_schema.columns
+        WHERE table_name = 'platform_steps'
+    """))
+    columns = {row[0] for row in result.fetchall()}
+    assert 'id' in columns, "platform_steps.id must exist"
+    assert 'account_id' not in columns, "platform_steps must not have account_id (global content)"