feat: add incident management and maintenance windows

Maintenance windows suppress alerts during planned downtime while checks
continue running. Incidents provide informational tracking. Supports
targeting all monitors, single monitor, or group (applies to children).

New Maint tab in TUI with create/end/delete. Status page, JSON API, and
Prometheus metrics all reflect maintenance state.

internal/store/sqlite.go

@@ -56,6 +56,17 @@ func (d *SQLiteDialect) CreateTablesSQL() []string {
 			message TEXT NOT NULL,
 			created_at DATETIME DEFAULT CURRENT_TIMESTAMP
 		)`,
+		`CREATE TABLE IF NOT EXISTS maintenance_windows (
+			id INTEGER PRIMARY KEY AUTOINCREMENT,
+			monitor_id INTEGER DEFAULT 0,
+			title TEXT NOT NULL,
+			description TEXT DEFAULT '',
+			type TEXT DEFAULT 'maintenance',
+			start_time DATETIME NOT NULL,
+			end_time DATETIME,
+			created_by TEXT DEFAULT '',
+			created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+		)`,
 	}
 }
 
@@ -96,6 +107,8 @@ func (d *SQLiteDialect) ImportWipe(tx *sql.Tx) {
 	tx.Exec("DELETE FROM sqlite_sequence WHERE name='alerts'")
 	tx.Exec("DELETE FROM users")
 	tx.Exec("DELETE FROM sqlite_sequence WHERE name='users'")
+	tx.Exec("DELETE FROM maintenance_windows")
+	tx.Exec("DELETE FROM sqlite_sequence WHERE name='maintenance_windows'")
 }
 
 func (d *SQLiteDialect) ImportResetSequences(tx *sql.Tx) {}