perf: queue-tuning and post-identity schema probes cost ~11 queries per request #3711

Problem

After #3708, these schema probes still run on every request, on every site:

• datamachine_initialize_queue_tuning → GroupRegistrar::tablesExist() → Action Scheduler tables_exist(): 4 queries. GroupRegistrar::ensureDataMachineGroup → ensureCustomTableGroup (table check plus group SELECT): 2 queries.
• datamachine_maybe_install_post_identity_reservations (plugins_loaded 5) runs PostIdentityReservations->validate_schema() (SHOW TABLES, SHOW COLUMNS, SHOW INDEX) before checking the datamachine_post_identity_reservations_schema option: 5 queries.

Desired state

• Both probes are gated by stored state, so a steady-state request does no schema queries:
  • the queue-tuning probes, once the AS tables and the Data Machine group are confirmed;
  • the reservations install, by checking the stored schema version first.
• Safety is preserved. Reservation writes already fail closed (Make post identity reservations crash-safe #2972). Keep that guarantee by revalidating, and clearing the option, when a reservation write hits a missing table or column, rather than revalidating on every request. Document the reasoning in the PR.
• Tests cover the gated no-op path, first install, and recovery after the table disappears.

Measured with a read-only full-page render profile on production (SAVEQUERIES, Redis object cache warm; tracking in Extra-Chill/extrachill#428). Follow-up to #3708 (PR #3709), which removes the 96-query ensure_all_tables() convergence; these are the next-largest per-request query sources.

🤖 Extra Chill Bot