Add SMS consent logging for Twilio compliance

Log phone, IP, and user agent to sms_consents table before sending
verification SMS. Add disclosure text to login form.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

migrate.go

@@ -22,6 +22,14 @@ func runMigrations(database *sql.DB) {
 		`DELETE FROM rsvps WHERE id NOT IN (SELECT MIN(id) FROM rsvps GROUP BY event_id, name COLLATE NOCASE)`,
 		// Plus-one tracking for RSVPs.
 		`ALTER TABLE rsvps ADD COLUMN plus_one INTEGER NOT NULL DEFAULT 0`,
+		// SMS consent logging for Twilio compliance.
+		`CREATE TABLE IF NOT EXISTS sms_consents (
+			id         INTEGER PRIMARY KEY AUTOINCREMENT,
+			phone      TEXT    NOT NULL,
+			ip_address TEXT    NOT NULL DEFAULT '',
+			user_agent TEXT    NOT NULL DEFAULT '',
+			created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+		)`,
 	}
 	for _, m := range migrations {
 		_, err := database.Exec(m)