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/postgres.go

@@ -56,6 +56,17 @@ func (d *PostgresDialect) CreateTablesSQL() []string {
 			message TEXT NOT NULL,
 			created_at TIMESTAMP DEFAULT NOW()
 		)`,
+		`CREATE TABLE IF NOT EXISTS maintenance_windows (
+			id SERIAL PRIMARY KEY,
+			monitor_id INTEGER DEFAULT 0,
+			title TEXT NOT NULL,
+			description TEXT DEFAULT '',
+			type TEXT DEFAULT 'maintenance',
+			start_time TIMESTAMP NOT NULL,
+			end_time TIMESTAMP,
+			created_by TEXT DEFAULT '',
+			created_at TIMESTAMP DEFAULT NOW()
+		)`,
 	}
 }
 
@@ -87,10 +98,12 @@ func (d *PostgresDialect) ImportWipe(tx *sql.Tx) {
 	tx.Exec("TRUNCATE TABLE sites RESTART IDENTITY CASCADE")
 	tx.Exec("TRUNCATE TABLE alerts RESTART IDENTITY CASCADE")
 	tx.Exec("TRUNCATE TABLE users RESTART IDENTITY CASCADE")
+	tx.Exec("TRUNCATE TABLE maintenance_windows RESTART IDENTITY CASCADE")
 }
 
 func (d *PostgresDialect) ImportResetSequences(tx *sql.Tx) {
 	tx.Exec("SELECT setval('sites_id_seq', (SELECT COALESCE(MAX(id), 1) FROM sites))")
 	tx.Exec("SELECT setval('alerts_id_seq', (SELECT COALESCE(MAX(id), 1) FROM alerts))")
 	tx.Exec("SELECT setval('users_id_seq', (SELECT COALESCE(MAX(id), 1) FROM users))")
+	tx.Exec("SELECT setval('maintenance_windows_id_seq', (SELECT COALESCE(MAX(id), 1) FROM maintenance_windows))")
 }