fix: serialize UserRole as lowercase in JWT

Add #[serde(rename_all = "lowercase")] to UserRole enum so JWT payloads
contain lowercase role strings ("guest", "host", "admin") matching what
the frontend route guards and getRole() comparisons expect.

Without this, JWTs contained PascalCase ("Guest", "Host", "Admin")
causing hosts and admins to be redirected to /join instead of their
dashboards.

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

backend/src/models/user.rs

@@ -4,6 +4,7 @@ use sqlx::PgPool;
 use uuid::Uuid;
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, sqlx::Type)]
+#[serde(rename_all = "lowercase")]
 #[sqlx(type_name = "user_role", rename_all = "lowercase")]
 pub enum UserRole {
     Guest,