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/auth/mod.rs

@@ -28,7 +28,7 @@ async fn verify_api_token(state: &AppState, raw_token: &str, client_ip: Option<S
     let token_hash = hex::encode(Sha256::digest(raw_token.as_bytes()));
 
     let row: Option<(String, Option<String>, String)> = sqlx::query_as(
-        "SELECT account_id, expires_at, permissions FROM api_tokens
+        "SELECT account_id, expires_at::TEXT, permissions FROM api_tokens
          WHERE token_hash = $1 AND revoked_at IS NULL"
     )
     .bind(&token_hash)