feat(privacy): WP-41 — persisted, queryable authz/PII-reveal audit

Persist the security-relevant events (authz denials + BIG-nummer reveal/step-up) into a
data-minimised EF table (AuthzAuditEntry: At/Action/Resource/Decision/Role/CorrelationId —
never a name/BSN/value), extending the DocumentStore AuditEntry pattern (migration AuthzAudit).
AuditAuthz now persists via AuthzAuditStore.Record alongside its log line. GET /admin/audit
(admin-gated by the existing CasesAdmin) returns the trail newest-first. +3 backend tests incl.
a schema-carries-no-PII reflection test. Typed client regenerated (audit() + AuthzAuditDto);
no FE consumer yet (a future audit view must add the ROLE_AWARE prefix). Finishes WP-42's audit half.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/BigRegister.Api/Data/Migrations/AppDbContextModelSnapshot.cs

@@ -99,6 +99,40 @@ namespace BigRegister.Api.Data.Migrations
                     b.ToTable("AuditEntries");
                 });
 
+            modelBuilder.Entity("BigRegister.Api.Data.AuthzAuditEntry", b =>
+                {
+                    b.Property<long>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("Action")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("At")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("CorrelationId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Decision")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Resource")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Role")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("AuthzAudit");
+                });
+
             modelBuilder.Entity("BigRegister.Api.Data.BriefEntity", b =>
                 {
                     b.Property<string>("BriefId")