BUGFIX: EventParticipantSecretV2Service - Override Redacted Secret From Storage On Modify

[cjdutoit]

Fixes a regression introduced by the #13 secret-hash redaction (#726). Once the client redacts Secret to null on reads, the foundation Modify rejected it — the structural "Secret required" rule and the against-storage "must match" rule both fail on a null secret. The Operations Portal re-reads the redacted secret and passes it to Modify, so editing a secret's lifecycle (activate/deactivate, active-from/to) failed with "Unable to save the secret." — reproduced end-to-end.

Fix

• Foundation — Modify no longer requires Secret, and ModifyEventParticipantSecretV2Async overrides the incoming Secret with the stored value before the against-storage validation. The secret stays immutable (any supplied value is ignored), lifecycle-only edits succeed, and the must-match-storage rule passes automatically.
• Guard test — the previous "SecretNotSameAsStorage throws" test is repurposed to assert the override (a differing secret is overridden and Modify succeeds); if the override is ever removed, this test flips back to a failure and catches it.
• Portal — the participant secret grid renders ***** for the redacted column (Admin + My Account pages); the Edit flow is unchanged.

Verification

• 3028 unit + 63 acceptance (SQL Server + Postgres in CI) + 130 Portal tests green.
• End-to-end in the Portal: deactivated then reactivated a secret — both saved with no error, ***** shown throughout.

closes #727