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/conftest.py

@@ -55,6 +55,15 @@ async def test_db() -> AsyncGenerator[AsyncSession, None]:
         await conn.execute(sa.text("CREATE SCHEMA public"))
         await conn.run_sync(Base.metadata.create_all)
 
+        # Seed plan_limits for subscription checks
+        await conn.execute(sa.text("""
+            INSERT INTO plan_limits (plan, max_trees, max_sessions_per_month, max_users, custom_branding, priority_support, export_formats)
+            VALUES
+                ('free', 3, 20, 1, false, false, '["markdown", "text"]'),
+                ('pro', 25, 200, 5, true, false, '["markdown", "text", "html"]'),
+                ('team', NULL, NULL, NULL, true, true, '["markdown", "text", "html"]')
+        """))
+
     # Create async session maker
     async_session_maker = async_sessionmaker(
         engine,