Set 30-day default time range for Alerts entity tables

[kmerz]

What?

The Alerts page histogram always defaults to a fixed time range of the last 6 months (180 days). However, the Graylog Events index set is commonly configured with a 30-day rotation. As a result, the histogram mostly displays empty data (≈5 months empty, 1 month populated).

Introduce a default 30-day time range filter for entity tables on the Alerts page. This should be configurable via the existing entity table filters, allowing users to quickly adjust the time range with a single click.

Why?

The current default produces a misleading, noisy visualization, showing large, empty periods that do not reflect the actual retained data. A 30-day default aligns better with typical Events index rotations and improves usability while still allowing users to expand the time range easily when needed.