Busy page

Purpose: Tell a visitor quickly that the site cannot answer right now, and retry for them, instead of leaving the page loading. Scenario: A full rebuild holds the read model locked. A visitor opens a thread; after waiting at most 2 seconds for the read model, the page answers 503 with the site profile's busy copy, Retry-After: 10 and a 10-second reload, and loads normally once the lock is gone. After a deploy that changes the read-model schema, the same visitor gets the "Site Update" page in the same form until the queued rebuild finishes. Sources: front controller (source unavailable) (renderBusyError(), renderReadModelRecovery(), sendRetryablePage()), wait limit (source unavailable) (PAGE_REQUEST_WAIT_MILLISECONDS, isLocked()), unavailable read model (source unavailable), copy (source unavailable) (busy()), tests (source unavailable).

Boundaries: page requests only. API requests keep their JSON busy response with Retry-After: 10, and so do the Deploy page's script requests (X-Requested-With: ForumDeploy), which keep checking status through a 503 instead of stopping; CLI commands and writers keep SQLite's default 60-second wait.