Feature Request: Audit logging for successful personal vault logins

[potfile0]

Summary

Vaultwarden currently does not log successful personal vault login
events, creating a significant security audit gap.

Current Behavior

The following events ARE logged:

• Failed admin token attempts
• Rate limiting events
• Registration attempts
• Organizational events (via existing audit log feature Add Organizational event logging feature #2868)

The following are NOT logged:

• Successful personal vault logins
• Which vault items were accessed/viewed
• Vault item modifications by authenticated users

Security Impact

Without successful login logging:

• Cannot detect unauthorized account access after the fact
• Cannot investigate security incidents meaningfully
• No audit trail for compliance requirements (GDPR, SOC2, HIPAA)
• Attacker can access vault repeatedly with zero evidence in logs
• The most sensitive service in a self hosted stack produces
  zero evidence of successful unauthorized access

Proposed Solution

In src/api/identity.rs, after successful authentication,
add a log entry:

log::info!(
    "Successful vault login for user: {} from IP: {}", 
    user_email, 
    client_ip
);

Optionally configurable via an environment variable:

LOG_SUCCESSFUL_LOGINS=true/false

Additional Context

This gap was identified during a STRIDE threat model analysis
of a self hosted Vaultwarden deployment. The Repudiation threat
category specifically highlighted that the most sensitive service
in the infrastructure produces zero forensic evidence of
successful unauthorized access.

I would be happy to contribute a PR implementing this feature
after learning more about the Vaultwarden codebase.

References

• Existing organizational audit log: Add Organizational event logging feature #2868
• STRIDE threat modelling methodology
• GDPR Article 32 (security of processing)