fix(outbox-dedup): buildSchedulerGroupPayload typed-nil broke dedup_key consistency

#3255 introduced payload-aware dedup_key (sha256 over event_type, account_id,
group_id, payload_json). enqueueSchedulerOutbox checks "if payload != nil"
before json.Marshal — but Go interface containing a typed-nil map (returned
by buildSchedulerGroupPayload(empty)) is NOT == nil at the interface level.

So an ungrouped account's account_changed event went through this path:
  payload := buildSchedulerGroupPayload(account.GroupIDs)  // typed-nil map
  enqueueSchedulerOutbox(..., payload)                     // interface != nil
    → json.Marshal(typedNilMap) = "null"
    → dedup_key hash = sha256(... + "null")

While other call sites pass literal nil:
  enqueueSchedulerOutbox(..., nil)                         // interface == nil
    → payloadJSON stays empty
    → dedup_key hash = sha256(... + "")

The two dedup_keys differ for what should be the same logical event,
silently degrading dedup effectiveness in bursts on ungrouped accounts.

Fix: change buildSchedulerGroupPayload return type from map[string]any to
any so empty input returns true untyped-nil. All call sites pass the
result straight to enqueueSchedulerOutbox(payload any) — no inspection,
no breakage.

Adds regression test TestEnqueueSchedulerOutbox_UngroupedAccountDedupesWithLiteralNilPayload
asserting (1) typed-nil regression doesn't sneak back, (2) dedup_key for
empty-groups payload matches the literal-nil-payload key.

backend/internal/repository/account_repo.go

@@ -1882,7 +1882,11 @@ func mergeGroupIDs(a []int64, b []int64) []int64 {
 	return out
 }
 
-func buildSchedulerGroupPayload(groupIDs []int64) map[string]any {
+// buildSchedulerGroupPayload 构造 EventAccountChanged / EventAccountGroupsChanged
+// 事件的 payload。空 groupIDs 必须返回 untyped nil（any 而非 map[string]any(nil)），
+// 否则 enqueueSchedulerOutbox 的 "payload != nil" 接口判空会被 typed-nil 欺骗，
+// 把 payload marshal 成 "null" 写入 dedup_key 哈希，破坏与其他 nil-payload 调用的去重一致性。
+func buildSchedulerGroupPayload(groupIDs []int64) any {
 	if len(groupIDs) == 0 {
 		return nil
 	}

backend/internal/repository/scheduler_outbox_repo_test.go

@@ -90,3 +90,29 @@ func TestSchedulerOutboxRepositoryTryAcquireCleanupLockUnavailable(t *testing.T)
 
 	require.NoError(t, mock.ExpectationsWereMet())
 }
+
+// buildSchedulerGroupPayload 在 groupIDs 为空时必须返回 untyped nil（any），
+// 否则 enqueueSchedulerOutbox 的 "payload != nil" 接口判空会被 typed-nil 欺骗，
+// 把 payload marshal 成 "null" 写入 dedup_key 哈希，破坏与其他 nil-payload
+// 调用的去重一致性。本测试用 ungrouped 账号场景验证两条路径的 dedup_key 一致。
+func TestEnqueueSchedulerOutbox_UngroupedAccountDedupesWithLiteralNilPayload(t *testing.T) {
+	accountID := int64(42)
+
+	// Path A: 显式 nil payload（如 SetError、SetStatus 等调用模式）
+	keyLiteralNil := schedulerOutboxDedupKey("account_changed", &accountID, nil, nil)
+
+	// Path B: buildSchedulerGroupPayload(account.GroupIDs) 当账号没有任何分组
+	emptyGroupsPayload := buildSchedulerGroupPayload(nil)
+	require.Nil(t, emptyGroupsPayload,
+		"buildSchedulerGroupPayload(empty) must return untyped-nil any to avoid typed-nil marshal")
+
+	// 模拟 enqueueSchedulerOutbox 内部的判空逻辑
+	var payloadJSON []byte
+	if emptyGroupsPayload != nil {
+		t.Fatalf("typed-nil regression: buildSchedulerGroupPayload(empty) interface should be nil")
+	}
+	keyEmptyGroups := schedulerOutboxDedupKey("account_changed", &accountID, nil, payloadJSON)
+
+	require.Equal(t, keyLiteralNil, keyEmptyGroups,
+		"ungrouped-account account_changed must share dedup_key with other nil-payload variants")
+}