Add JWT-based login flow with key pair generation

Introduced a secure login flow using JWTs with dynamically generated RSA key pairs stored in the database. Updated relevant APIs, database schema, and services to support login codes, JWT encoding, and secret validation.

backend/src/main/resources/application.properties

@@ -5,4 +5,5 @@ database.host=${DB_HOST:localhost}
 database.user=${DB_USER:root}
 database.password=${DB_PASSWORD:root}
 cors.allowed-origins=${CORS:https://alttd.com}
+login.secret=${LOGIN_SECRET:SET_TOKEN}
 logging.level.com.alttd.altitudeweb=INFO