feat(tui,status): add per-site pause, fix viewport, polish status page

Per-site pause: [p] key toggles pause for selected monitor in TUI.
Paused monitors skip checks, persist to DB, show on status page.

Status page: replace full-page reload with fetch-based DOM updates
to eliminate scroll-jump on refresh. Add summary bar (UP/DOWN/PAUSED
counts), stale-data indicator, and fix SSL EXP CSS class bug.

TUI: constrain tables to terminal width via lipgloss .Width() to
prevent row wrapping that pushed header off-screen. Add MaxHeight
safety net. Bump subtle style from #383838 to #565f89 for
readability on dark terminals.

internal/store/store.go

@@ -11,6 +11,7 @@ type Store interface {
 	GetSites() []models.Site
 	AddSite(site models.Site)
 	UpdateSite(site models.Site)
+	UpdateSitePaused(id int, paused bool)
 	DeleteSite(id int)
 
 	// Alerts