[FEATURE] add configurable cool-down option

[HannahVernon]

Which component(s) does this affect?

• Full Dashboard
• Lite
• SQL collection scripts
• Installer
• Documentation

Problem Statement

The Dashboard and Lite apps currently use a hard-coded "cool down" period which limits the frequency of pop-up alerts and email alerts. This feature adds a user-configurable period via the Settings windows in each app.

Proposed Solution

Plan for branch: feature/add-cooldown-period-setting

The cooldown is currently private static readonly TimeSpan AlertCooldown = TimeSpan.FromMinutes(5) in both MainWindow.xaml.cs files (Dashboard line 63, Lite line 46). It's used at 7 comparison sites in each edition (one per alert type: blocking, deadlock, CPU, poison waits, long-running queries, TempDB space, long-running jobs).

What changes

Dashboard — uses UserPreferences for all alert settings:

1. Dashboard/Models/UserPreferences.cs — add public int AlertCooldownMinutes { get; set; } = 5;
2. Dashboard/MainWindow.xaml.cs — remove the static readonly field; introduce var alertCooldown = TimeSpan.FromMinutes(prefs.AlertCooldownMinutes); as a local at the top of EvaluateAlertConditionsAsync (where prefs is already fetched), replace all 7 AlertCooldown usages with it
3. Dashboard/SettingsWindow.xaml — add AlertCooldownTextBox + label before the per-alert-type rows
4. Dashboard/SettingsWindow.xaml.cs — load from prefs.AlertCooldownMinutes, save with 1–60 validation, restore default = 5

Lite — uses App.xaml.cs static properties + JSON:

1. Lite/App.xaml.cs — add public static int AlertCooldownMinutes { get; set; } = 5;, load from alert_cooldown_minutes (clamped 1–60), save to JSON
2. Lite/MainWindow.xaml.cs — remove static readonly field; add var alertCooldown = TimeSpan.FromMinutes(App.AlertCooldownMinutes); at top of CheckPerformanceAlerts, replace all 7 usages
3. Lite/Windows/SettingsWindow.xaml — add AlertCooldownBox + label
4. Lite/Windows/SettingsWindow.xaml.cs — load/save/restore pattern matching existing settings

One important non-change: GetFilteredDeadlockCountAsync has its own 5-minute SQL window — that's a data collection window, completely separate from the notification cooldown, and stays hardcoded.

Use Case

Personally, I would like to limit the frequency of alerts to be either:

(a) shorter to allow easier testing of changes to the alerts code
(b) allow the user to limit emails/alert popups to once per hour or whatever they think is helpful for their situation.

Alternatives Considered

No response

Additional Context

No response

[HannahVernon]

The plan is being updated to allow for two separate configurable options, one for the alerts popups, and one for email alerts., with feature parity across apps.