Address production-readiness review: clearer errors, safer votes, and ops hardening.

Distinguish auth/lookup failures, make votes idempotent on visible questions, bound shutdown, page admin users, LRU throttle, trusted-proxy CIDRs, avatar cleanup, versioned migrations, and session cleanup logging.

internal/web/server_test.go

@@ -332,6 +332,8 @@ func (f *fakeBlob) Upload(_ context.Context, obj blob.FileUpload) (string, error
 	return "https://cdn.example.com/" + obj.Key, nil
 }
 
+func (f *fakeBlob) Delete(_ context.Context, _ string) error { return nil }
+
 func TestProfilePageAndState(t *testing.T) {
 	srv, mem := newTestServer(t, Config{})
 	h := srv.Handler()