The Received line was sent as a raw unix timestamp. It is now a business
configuration field (`timezone`, IANA name, default `Asia/Shanghai`), edited
in the admin SPA and applied through the existing 1s hot-reload.
The offline build has no IANA tz database (chrono-tz is unavailable), so the
server keeps a static table of 16 zones that carry no daylight saving
transition and rejects anything else with the existing 422 configuration
error instead of guessing an offset.
`format_received_at` renders to `%Y-%m-%d %H:%M` at notification time; a
timestamp outside the representable range falls back to `unknown`.
Chrono 0.4 was already in Cargo.lock as a jmap-client transitive dependency,
so this only promotes it to a direct dependency.
anonsaber