test(saas): Phase 1 integration tests — billing + scheduled_task + knowledge (68 tests)

- Fix TIMESTAMPTZ decode errors: add ::TEXT cast to all SELECT queries
  where Row structs use String for TIMESTAMPTZ columns (~22 locations)
- Fix Axum 0.7 route params: {id} → :id in billing/knowledge/scheduled_task routes
- Fix JSONB bind: scheduled_task INSERT uses ::jsonb cast for input_payload
- Add billing_test.rs (14 tests): plans, subscription, usage, payments, invoices
- Add scheduled_task_test.rs (12 tests): CRUD, validation, isolation
- Add knowledge_test.rs (20 tests): categories, items, versions, search, analytics, permissions
- Fix auth test regression: 6 tests were failing due to TIMESTAMPTZ type mismatch

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

crates/zclaw-saas/src/scheduler.rs

@@ -92,7 +92,7 @@ pub fn start_db_cleanup_tasks(db: PgPool) {
                 "DELETE FROM devices WHERE last_seen_at < $1"
             )
             .bind({
-                let cutoff = (chrono::Utc::now() - chrono::Duration::days(90)).to_rfc3339();
+                let cutoff = (chrono::Utc::now() - chrono::Duration::days(90));
                 cutoff
             })
             .execute(&db_devices)