Bound cron cleanup and index normalized suppression lookups

Problem/Motivation

Cron performs one unbounded DELETE and suppression queries apply LOWER() to recipient despite a plain recipient index. This can become expensive as retained event history grows; no throughput benchmark has established a current production threshold.

Evidence and scope

Reviewed 1.0.0-alpha1, source commit 02fd9d36af5237e712cecb7155d79725f7824880. Location: postmark_webhooks.module:48.

Source inspection only; performance risk, not a measured outage.

Proposed resolution

Benchmark realistic history sizes on supported databases, add portable indexed normalized lookup/state access, and make retention cleanup resumable in bounded batches. Keep legacy mixed-case correctness. Add queueing only if measured ingestion latency justifies its complexity and acknowledge only after durable storage.

Acceptance criteria

Record query plans and bounded runtime/memory, test concurrent cron and insertion, and prove batch boundaries do not alter suppression or idempotency. Avoid database-specific optimizations without a portable fallback.