perf(jmap): reuse one JMAP client per account within a process

[SebastianKrupinski]

Summary

• Every JMAP operation built a new client, repeating session discovery and the TLS handshake. Pool clients per account and replace them when connection settings or credentials change.
• SyncJob releases the account's clients so cron runs don't keep connections across accounts.
• IMAP pooling and job release points for IMAP-only jobs follow separately.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI (N/A)