Optional background snapshot preparation and snapshot-ready events

Problem/Motivation

The first reader after a comment change pays for rebuilding the snapshot. Preparing it in advance can improve refresh latency, but rebuilding after every web response adds unnecessary work on small sites.

Proposed resolution

• Keep core ACT demand-built. Make anonymous warming optional through the Background submodule.
• Collect affected host/field pairs transactionally, including old and new locations on moves. Coalesce writes across requests and prepare only configured snapshot variants.
• Protect rebuilds against concurrent writes and restore account switching reliably.
• Emit advanced_comment_threads.snapshot_ready only after a current snapshot has been successfully cached. Direct push updates need not wait for a full snapshot.
• Keep authenticated snapshots demand-built.

Remaining tasks

• Review the local implementation and draft ADR 0010.
• Verify coalescing, moves, rollback, concurrent writes, account restoration and subsequent anonymous cache hits.
• Verify that failed, disabled or uncacheable preparation never emits readiness.
• Measure and document write overhead and background rebuild cost.

User interface changes

No new UI. Background warming is opt-in through configuration.

API changes

Explicit snapshot preparation and a snapshot-ready event identifying the host, field, variant and generation.

Data model changes

Host generations use existing key/value storage. The optional Background submodule adds a transactional outbox.