fix(audit): store plugin entries with no acting user as a null user_id (#1556)

Plugin audit entries written outside a request used the literal "system"
as user_id. That column references users.id, so the insert was refused
(Postgres logs it as an FK violation) and the entry was silently dropped.
Write null instead and keep "system" / plugin:<id> in username.

src/backend/tests/plugins/ctx-audit.test.ts

@@ -73,7 +73,8 @@ describe("ctx.audit.record", () => {
     const ctx = contextFor("audit-fixture");
     await ctx.audit.record({ action: "cleanup", success: false });
     expect(auditEntries[0]).toMatchObject({
-      userId: "system",
+      userId: null,
+      username: "system",
       resourceType: "plugin",
       resourceId: "audit-fixture",
       resourceName: "Audit Fixture",