Alerts toned to evidence weight + page time picker drives the date filter

Two related Alerts changes:

• Severity follows the evidence. A 404 spike based on three requests now reads as Low, not Critical. The same anomaly seen across hundreds of requests still scores Critical. The thresholds were uncoupled from raw count and rebound to a confidence weight, so the badge actually tells you whether to act.
• One time picker. The Alerts page used to have its own date selector. It now reads from the page-header time picker — the same one Dashboard, Traffic, IP Analysis and Recommendations use — so switching from "Last 24 hours" to "Last 7 days" carries everywhere.