WebhookEvent, FeatureStateResult, ProjectPermissionRequirement, AuditLog,
Tag, and FeatureUsageDaily are plain data holders with no behavior, so
switch them to records. EF-mapped entities with identity semantics
(User, Organization, Project, Feature, Segment, etc.) stay classes,
since record value-equality and deep ToString fight EF's change tracker.
Also restore `virtual` on AuditService.RecordAsync/LogAsync, which had
been dropped and broke Mock<AuditService>-based test setups.
