Complete publish preview Phase 2A with retention, refresh and cache consistency

backend/docs/publish-preview-phase-2a-status.md

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+# Publish Preview — Phase 2A Status
+
+## Status
+
+Phase 2A is mostly closed.
+
+## Confirmed
+
+- retention policy works
+- `refresh=true` works
+- cached read works
+- response contract is more consistent
+- cleanup actually removes old records from the database
+
+## Retention result
+
+For the current draft version, exactly 2 preview artifacts remain in the database, matching the configured retention policy.
+
+## Response contract
+
+The response now explicitly returns:
+
+- `preview_cache_cleanup.retention=2`
+- `deleted_count`
+- `deleted_artifact_ids`
+
+## Runtime state
+
+- logs are clean
+- no new exceptions were observed