fix(sync-service): Prevent EtsInspector's mailbox from getting flooded when the cache is cold

.changeset/ets-inspector-mailbox-overload.md

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+---
+'@core/sync-service': patch
+---
+
+Stop the EtsInspector from overloading its mailbox and re-running database lookups during cold-start bursts or while Postgres is degraded (#4370):
+
+- Concurrent lookups of the same relation, oid, or feature now share a single database call instead of each running their own.
+- Failed lookups (table-not-found and connection errors) are cached briefly, so a burst against a failing key stops hammering the database.
+- Each lookup has an explicit transaction timeout.
+- Each lookup is recorded as an `inspector.fetch_db` span so its latency and outcome are visible in telemetry.