DeleteAccountHandler now scrubs external systems BEFORE SQL-level anonymization. S3: deletes profile picture from S3/R2 (profiles/{userID}.jpg) when s3.Client is configured. Square: iterates user's saved cards and calls DeleteCardOnFile for each non-deleted card when payments.SquareClient is configured. Both run in background goroutines (non-blocking, best-effort with warning logs). CardDAV contact deletion remains as existing best-effort goroutine.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>