feat: externalize Ed25519 keys with file-based loading

Keys are loaded from PEM files when CAMELEER_JWT_PRIVATE_KEY_PATH and
CAMELEER_JWT_PUBLIC_KEY_PATH are set. Falls back to ephemeral key
generation for development.

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

src/main/resources/application.yml

@@ -21,3 +21,5 @@ management:
 cameleer:
   jwt:
     expiration: 86400 # 24 hours in seconds
+    private-key-path: ${CAMELEER_JWT_PRIVATE_KEY_PATH:}
+    public-key-path: ${CAMELEER_JWT_PUBLIC_KEY_PATH:}