fix: Ed25519SigningService falls back to ephemeral key when jwt-secret is absent

- SecurityBeanConfig uses Ed25519SigningServiceImpl.ephemeral() when no jwt-secret
- Fixes pre-existing application context failure in integration tests
- Reverts test jwt-secret from application-test.yml (no longer needed)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/main/java/com/cameleer3/server/app/security/SecurityBeanConfig.java

@@ -22,7 +22,11 @@ public class SecurityBeanConfig {
 
     @Bean
     public Ed25519SigningServiceImpl ed25519SigningService(SecurityProperties properties) {
-        return new Ed25519SigningServiceImpl(properties.getJwtSecret());
+        String secret = properties.getJwtSecret();
+        if (secret == null || secret.isBlank()) {
+            return Ed25519SigningServiceImpl.ephemeral();
+        }
+        return new Ed25519SigningServiceImpl(secret);
     }
 
     @Bean

cameleer3-server-app/src/test/resources/application-test.yml

@@ -17,4 +17,3 @@ agent-registry:
 security:
   bootstrap-token: test-bootstrap-token
   bootstrap-token-previous: old-bootstrap-token
-  jwt-secret: test-jwt-secret-for-ed25519-derivation