Transaction Processing and Enrichment

You are tasked with building a system that processes a large number of transactions. Each transaction is represented as a hash containing the following fields:
* id: Unique identifier of the transaction.
* amount: The amount of money for the transaction.
* currency: The currency of the transaction (e.g., 'USD', 'EUR').
* user_id: Identifier of the user who made the transaction.
The system needs to:
1. Validate each transaction to ensure that:
    * The id is present and is a unique value.
    * The amount is a positive number.
    * The currency is a valid currency code (e.g., 'USD', 'EUR', 'GBP').
    * The user_id is present and corresponds to an existing user.
2. Enrich each valid transaction by adding two additional fields:
    * processed_at: The current timestamp when the transaction was processed.
    * enrichment_code: A random alphanumeric code generated for the transaction.
3. Save the enriched transaction to an in-memory simulated database (represented as an array).
4. The system should process a list of transactions and handle the following scenarios gracefully:
    * If a transaction is invalid (e.g., missing required fields or having incorrect data types), it should be skipped, and an appropriate error message should be logged.
    * If an error occurs while enriching a transaction (e.g., failure to generate a random code), the system should log the error and continue processing the remaining transactions.
    * The system should not crash if one transaction fails; it should continue processing the others.
5. Output the final list of enriched transactions.