Convert logic-free classes to records per CLAUDE.md

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.

src/api/MicCheck.Api/Features/FeatureStateResult.cs

@@ -1,6 +1,6 @@
 namespace MicCheck.Api.Features;
 
-public class FeatureStateResult
+public record FeatureStateResult
 {
     public required Feature Feature { get; init; }
     public bool Enabled { get; init; }