feat: update all endpoints and schemas for account-based model

Replace team_id with account_id across all API endpoints (trees,
categories, tags, steps, step_categories, admin, auth). Add new
accounts and webhooks endpoints. Registration now atomically creates
Account + Subscription, with account_invite_code bypassing the
platform invite gate.

Schemas updated for account_id/account_role. 82 tests passing
including 18 new tests for accounts, subscriptions, and permissions.

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

backend/tests/test_auth.py

@@ -23,6 +23,8 @@ class TestAuthentication:
         assert data["email"] == user_data["email"]
         assert data["name"] == user_data["name"]
         assert data["role"] == "engineer"
+        assert "account_id" in data
+        assert data["account_role"] == "owner"
         assert "id" in data
         assert "password" not in data  # Password should not be returned
 
@@ -107,6 +109,7 @@ class TestAuthentication:
         assert response.status_code == 201
         data = response.json()
         assert data["role"] == "engineer"
+        assert data["account_role"] == "owner"
 
     @pytest.mark.asyncio
     async def test_register_default_role_is_engineer(self, client: AsyncClient):
@@ -121,6 +124,7 @@ class TestAuthentication:
 
         assert response.status_code == 201
         assert response.json()["role"] == "engineer"
+        assert response.json()["account_role"] == "owner"
 
     @pytest.mark.asyncio
     async def test_register_rejects_no_uppercase(self, client: AsyncClient):