fix: revert broad metadata batching

[ctxswitch]

Summary

Narrow the bbolt batching rollback to the operations that caused the production lock contention, while retaining request-path batching controls and telemetry.

The failing production image changed serialized S3FIFO maintenance from immediate transactions to delayed bbolt.Batch calls. Eviction holds queueMu while scanning pinned entries, so every candidate paid the batch delay twice while the global admission lock remained held. Expiry was also changed from one write transaction per reaper batch to one delayed batch call per expired entry.

What changed

• Restore immediate bbolt.Update transactions for serialized S3FIFO eviction and ghost maintenance.
• Restore one write transaction per envelope-expiry batch.
• Keep batching only on the paths that already used it in the stable image: blob puts, blob touches, envelope puts, S3FIFO admissions, and ghost hits.
• Retain METADATA_BATCH_SIZE and METADATA_BATCH_DELAY configuration.
• Set defaults to the stable-image behavior: batch size 1000 and delay 1ms.
• Retain metadata batching and bbolt write telemetry.
• Add regressions proving eviction maintenance does not wait for the request batch window and expiry batches commit in one write transaction.

Compatibility

No data-format or migration impact. The configuration knobs remain available. Their defaults preserve the effective behavior of the previous production image: the service explicitly used a 1 ms delay and inherited bbolt v1.5.0 batch size 1000.

Configuration

Option	Environment variable	Default
Metadata batch size	METADATA_BATCH_SIZE	1000
Metadata batch delay	METADATA_BATCH_DELAY	1ms

These settings affect only operations that use request-path batching. Serialized eviction maintenance and bulk expiry cleanup do not wait on this batch window.

Validation

• Regression tests: TestPopTailDoesNotWaitForRequestBatchWindow and TestEnvelopeReaperDeletesEachBatchInSingleWriteTransaction
• Full suite: GOCACHE=/tmp/codex-go-build go test ./...
• gofmt and git diff --check