feat: add password support for local user creation and per-user login

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

cameleer3-server-core/src/main/java/com/cameleer3/server/core/security/UserRepository.java

@@ -15,4 +15,8 @@ public interface UserRepository {
     void upsert(UserInfo user);
 
     void delete(String userId);
+
+    void setPassword(String userId, String passwordHash);
+
+    Optional<String> getPasswordHash(String userId);
 }