fix: harden message seq allocation against stale redis replicas

- use sentinel failover client for group message runtime when
  sentinel_master_name/sentinel_addrs are configured, so seq counter
  reads never hit a stale replica after failover
- re-seed missing redis seq counter from DB topic seqid instead of
  trusting the caller in-memory baseline
- on unique-key conflict during async persist, re-allocate seq from DB
  latest, bump redis counter and fix the idempotency record so resends
  with the same client mid return the final seq
- add uk(topic, seqid) unique keys for im_messages in tenant DDL

server/store/group_message_runtime_test.go

@@ -43,6 +43,17 @@ func (r *testGroupMessageRuntime) NextSeq(_ types.TenantID, topic string, baseli
 	return r.seqs[topic], nil
 }
 
+func (r *testGroupMessageRuntime) PeekSeq(_ types.TenantID, topic string) (int64, error) {
+	return r.seqs[topic], nil
+}
+
+func (r *testGroupMessageRuntime) BumpSeqFloor(_ types.TenantID, topic string, seq int64) error {
+	if r.seqs[topic] < seq {
+		r.seqs[topic] = seq
+	}
+	return nil
+}
+
 func (r *testGroupMessageRuntime) MarkDone(_ types.TenantID, topic string, from types.Uid, clientMsgID string, seq int64, msgID string) error {
 	key := topic + ":" + from.UserId() + ":" + clientMsgID
 	r.claims[key] = &groupMessageUniqueRecord{Status: "done", Seq: seq, MessageID: msgID, CreatedAt: time.Now().UTC()}
@@ -246,6 +257,37 @@ func TestMessageRuntimeSeqAndClientMidIdempotency(t *testing.T) {
 	}
 }
 
+func TestBumpMessageSeqFloorNeverMovesBackward(t *testing.T) {
+	oldRuntime := groupMsgRuntime
+	runtime := newTestGroupMessageRuntime()
+	groupMsgRuntime = runtime
+	t.Cleanup(func() { groupMsgRuntime = oldRuntime })
+
+	tenantID := types.TenantID(7)
+	topic := types.Uid(1).P2PName(types.Uid(2))
+
+	seq, ready, err := ReserveMessageSeq(tenantID, topic, 0)
+	if err != nil || !ready || seq != 1 {
+		t.Fatalf("unexpected first seq: seq=%d ready=%v err=%v", seq, ready, err)
+	}
+	if err = BumpMessageSeqFloor(tenantID, topic, 5); err != nil {
+		t.Fatalf("bump floor: %v", err)
+	}
+	if current, err := runtime.PeekSeq(tenantID, topic); err != nil || current != 5 {
+		t.Fatalf("floor was not applied: current=%d err=%v", current, err)
+	}
+	if err = BumpMessageSeqFloor(tenantID, topic, 3); err != nil {
+		t.Fatalf("lower bump: %v", err)
+	}
+	if current, err := runtime.PeekSeq(tenantID, topic); err != nil || current != 5 {
+		t.Fatalf("floor must never move backward: current=%d err=%v", current, err)
+	}
+	// The next allocation must continue above the bumped floor.
+	if seq, _, err := ReserveMessageSeq(tenantID, topic, 0); err != nil || seq != 6 {
+		t.Fatalf("unexpected seq after bump: seq=%d err=%v", seq, err)
+	}
+}
+
 func TestInvalidateGroupMemberPagesUsesTopicKeyPrefix(t *testing.T) {
 	oldRuntime := groupMsgRuntime
 	runtime := newTestGroupMessageRuntime()