fix(api/purchases): GET planned purchases returns 500 (GetPlannedExecutions SELECT omits scheduled_execution_at) (QA row 579)

[cristim]

Symptom

On the Plans page, the "Planned Purchases" section fails to load with "Failed to load planned purchases: Internal server error" (HTTP 500). Reproducible by the Admin user, so this is not a permissions issue.

Root Cause

GetPlannedExecutions in internal/config/store_postgres.go projects only 26 columns in its SELECT, omitting scheduled_execution_at. The shared scanExecutionRows helper scans 27 columns, with &scheduledExecutionAt as the final (27th) scan target. Against a real PostgreSQL instance, the row Scan fails with "expected 27 destination arguments in Scan, not 26", returning "failed to scan execution" - which the handler surfaces as a 500.

This drifted in via PR #804 (revocation-delay feature, commit 8e5f139) which added scheduled_execution_at to scanExecutionRows and every other SELECT feeding it, but skipped GetPlannedExecutions.

The existing test TestPGXMock_GetPlannedExecutions_ProjectsAllScanColumns was a false positive: its mock ExpectQuery regexp only checked the SQL contains "idempotency_key" and returned a 27-column result regardless of the actual projection, so it passed even with the bug present.

Fix

1. Add scheduled_execution_at to the GetPlannedExecutions SELECT in the correct position (after idempotency_key, matching scanExecutionRows order).
2. Tighten the regression test's regexp to require both idempotency_key AND scheduled_execution_at appear in the SQL, so a future column drift is caught immediately.