NM-256: fix DB performance degradation with large user base and Entra IDP

- Replace full table scan in FetchRecord with indexed single-key lookup
      (SELECT WHERE key = ?) for PostgreSQL, SQLite, and RQLite backends
    - Add in-memory user cache (gated behind CACHING_ENABLED) to eliminate
      DB round-trips on the auth hot path (GetUser called per API request)
    - Configure PostgreSQL connection pool limits (max open/idle conns,
      conn lifetime) to prevent connection churn under load
    - Add periodic cleanup of expired SSO state entries to prevent
      unbounded table growth
    - Route GitHub OAuth user rename through standard logic functions
      to keep user cache consistent

database/sqlite.go

@@ -20,6 +20,7 @@ var SQLITE_FUNCTIONS = map[string]interface{}{
 	DELETE:       sqliteDeleteRecord,
 	DELETE_ALL:   sqliteDeleteAllRecords,
 	FETCH_ALL:    sqliteFetchRecords,
+	FETCH_ONE:    sqliteFetchRecord,
 	CLOSE_DB:     sqliteCloseDB,
 	isConnected:  sqliteConnected,
 }
@@ -103,6 +104,18 @@ func sqliteDeleteAllRecords(tableName string) error {
 	return nil
 }
 
+func sqliteFetchRecord(tableName string, key string) (string, error) {
+	var value string
+	err := SqliteDB.QueryRow("SELECT value FROM "+tableName+" WHERE key = ?", key).Scan(&value)
+	if err != nil {
+		if err == sql.ErrNoRows {
+			return "", errors.New(NO_RECORD)
+		}
+		return "", err
+	}
+	return value, nil
+}
+
 func sqliteFetchRecords(tableName string) (map[string]string, error) {
 	row, err := SqliteDB.Query("SELECT * FROM " + tableName + " ORDER BY key")
 	if err != nil {