Preserve created_at on user upsert to avoid accumulating un-merged rows

On re-login the upsert was inserting a new row with created_at=now(),
causing ClickHouse ReplacingMergeTree to accumulate rows until
background compaction. Now preserves the original created_at via
INSERT...SELECT from the existing record.

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

cameleer3-server-app/src/main/java/com/cameleer3/server/app/storage/ClickHouseUserRepository.java

@@ -47,14 +47,32 @@ public class ClickHouseUserRepository implements UserRepository {
 
     @Override
     public void upsert(UserInfo user) {
-        jdbc.update(
-                "INSERT INTO users (user_id, provider, email, display_name, roles, updated_at) VALUES (?, ?, ?, ?, ?, now64(3, 'UTC'))",
-                user.userId(),
-                user.provider(),
-                user.email(),
-                user.displayName(),
-                user.roles().toArray(new String[0])
-        );
+        // Preserve created_at from existing record on re-login.
+        // ReplacingMergeTree deduplicates by ORDER BY (user_id), keeping the row
+        // with the highest updated_at.
+        Optional<UserInfo> existing = findById(user.userId());
+        if (existing.isPresent()) {
+            jdbc.update(
+                    "INSERT INTO users (user_id, provider, email, display_name, roles, created_at, updated_at) "
+                            + "SELECT user_id, ?, ?, ?, ?, created_at, now64(3, 'UTC') "
+                            + "FROM users FINAL WHERE user_id = ?",
+                    user.provider(),
+                    user.email(),
+                    user.displayName(),
+                    user.roles().toArray(new String[0]),
+                    user.userId()
+            );
+        } else {
+            jdbc.update(
+                    "INSERT INTO users (user_id, provider, email, display_name, roles, updated_at) "
+                            + "VALUES (?, ?, ?, ?, ?, now64(3, 'UTC'))",
+                    user.userId(),
+                    user.provider(),
+                    user.email(),
+                    user.displayName(),
+                    user.roles().toArray(new String[0])
+            );
+        }
     }
 
     @Override