fix: sync conflict w.r.t. username

[uwwint]

Description

Improve the resilience of scheduled Auth0-to-database sync jobs so a single bad user record no longer breaks the entire sync run.

Changes

• Added guardrails in user sync to detect duplicate username/email conflicts before database flush.
• Changed sync flow to skip only the conflicting user and continue processing everyone else.
• Improved scheduler error logging so failures show real exception details and traceback.
• Added test coverage for the conflict scenario to prevent regression.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have added unit / integration tests that prove my fix is effective or that my feature works
• I have run all tests locally and they pass
• I have updated the documentation (if applicable)
• For any new secrets, I have updated the shared spreadsheet and the GitHub Secrets.

How to Test Manually (if necessary)

1. Trigger scheduled sync jobs (platform/group role sync).
2. Include a known conflicting Auth0 user (same username/email already used in DB by another user).
3. Confirm the job logs a warning for that user, skips it, and still completes successfully for others.
4. Confirm scheduler error logs print readable exception details (no placeholder text).

[amandazhuyilan]

Good looks to me but will leave @marius-mather to take a final lookie.

[marius-mather]

all looks good to me