fix(saas): migration idempotency fixes + SCHEMA_VERSION bump to 14

- Add IF NOT EXISTS to accounts_template_assignment ALTER COLUMN
- Add IF NOT EXISTS to webhooks CREATE INDEX statements
- Add created_at/updated_at columns + ON CONFLICT DO NOTHING to industry templates
- Bump SCHEMA_VERSION 13→14 to force migration re-run on existing DB

crates/zclaw-saas/migrations/20260403000001_accounts_template_assignment.sql

@@ -2,7 +2,7 @@
 -- 用户选择行业模板后记录分配关系，用于跟踪和跳过 onboarding
 -- ON DELETE SET NULL: 模板被删除时不影响账户
 
-ALTER TABLE accounts ADD COLUMN assigned_template_id TEXT
+ALTER TABLE accounts ADD COLUMN IF NOT EXISTS assigned_template_id TEXT
   REFERENCES agent_templates(id) ON DELETE SET NULL;
 
 COMMENT ON COLUMN accounts.assigned_template_id IS