Pushover rejects priority-2 (Emergency) messages unless they carry retry
and expire. _send_pushover never sent them, so setting priority 2 always
failed with Pushover's "retry and expire are required" error. Now at
priority 2 we send retry/expire (default 60s/3600s, clamped to Pushover's
30-10800s range), surfaced as two provider fields shown only when priority
is 2. Added PushoverConfig schema fields, i18n labels across all locales,
and unit tests.
