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/Audit/AuditService.cs

@@ -33,7 +33,7 @@ public class AuditService(IMicCheckDbContext db, IHttpContextAccessor httpContex
             }, JsonOptions);
         }
 
-        var actorUserId = ResolveActorUserId();
+        var actorUserId = GetCurrentUserId();
 
         var log = new AuditLog
         {
@@ -78,7 +78,7 @@ public class AuditService(IMicCheckDbContext db, IHttpContextAccessor httpContex
         string? changes = null,
         CancellationToken ct = default)
     {
-        var actorUserId = ResolveActorUserId();
+        var actorUserId = GetCurrentUserId();
 
         var log = new AuditLog
         {
@@ -112,7 +112,7 @@ public class AuditService(IMicCheckDbContext db, IHttpContextAccessor httpContex
         });
     }
 
-    private int? ResolveActorUserId()
+    private int? GetCurrentUserId()
     {
         var claim = httpContextAccessor.HttpContext?.User.FindFirst(System.Security.Claims.ClaimTypes.NameIdentifier)?.Value;
         return int.TryParse(claim, out var id) ? id : null;