[alerter]bugfix: preserve group alert recovery events

[hutiefang76]

What's changed

Close #4160.

This fixes two group alert lifecycle cases:

• do not let the firing repeat interval suppress pending resolved alerts in a mixed firing group
• recompute the persisted group status from all known member alerts, so one recovered alert does not mark the whole group resolved while another member is still firing

Regression tests cover both the in-memory group reducer path and the database store path.

Verification

• JAVA_HOME=$(/usr/libexec/java_home -v 25 2>/dev/null || /usr/libexec/java_home) ./mvnw -pl hertzbeat-alerter -am test
• git diff --check