fix: use cursor-based query for chat stream notifications (#22510)

## Problem

The pubsub notification handler in `chatd` re-fetched **all** messages
from the DB on every new message notification, then filtered in Go with
`msg.ID > lastMessageID`. This grows linearly with conversation length —
every new message triggers a full table scan of that chat's history.

The `AfterMessageID` field in the pubsub notification payload was
clearly designed for cursor-based fetching, but no matching query
existed.

## Fix

- Add `GetChatMessagesByChatIDAfter` SQL query with `WHERE id >
@after_id`, so the database does the filtering instead of Go.
- Use it in the pubsub notification handler in `chatd.go`, passing
`lastMessageID` as the cursor.
- Implement the dbauthz wrapper (was a `panic("not implemented")` stub
from codegen) with the same read-check-on-parent-chat pattern as
adjacent methods.
- Add dbauthz test coverage for the new method.

**Not changed:** The initial snapshot in `Subscribe()` still loads all
messages — that's correct, since a newly-connecting client needs the
full conversation state. The waste was only in the ongoing notification
path.

coderd/database/dbauthz/dbauthz.go

@@ -2465,13 +2465,13 @@ func (q *querier) GetChatMessageByID(ctx context.Context, id int64) (database.Ch
 	return msg, nil
 }
 
-func (q *querier) GetChatMessagesByChatID(ctx context.Context, chatID uuid.UUID) ([]database.ChatMessage, error) {
+func (q *querier) GetChatMessagesByChatID(ctx context.Context, arg database.GetChatMessagesByChatIDParams) ([]database.ChatMessage, error) {
 	// Authorize read on the parent chat.
-	_, err := q.GetChatByID(ctx, chatID)
+	_, err := q.GetChatByID(ctx, arg.ChatID)
 	if err != nil {
 		return nil, err
 	}
-	return q.db.GetChatMessagesByChatID(ctx, chatID)
+	return q.db.GetChatMessagesByChatID(ctx, arg)
 }
 
 func (q *querier) GetChatMessagesForPromptByChatID(ctx context.Context, chatID uuid.UUID) ([]database.ChatMessage, error) {

coderd/database/dbauthz/dbauthz_test.go

@@ -473,9 +473,10 @@ func (s *MethodTestSuite) TestChats() {
 	s.Run("GetChatMessagesByChatID", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		chat := testutil.Fake(s.T(), faker, database.Chat{})
 		msgs := []database.ChatMessage{testutil.Fake(s.T(), faker, database.ChatMessage{ChatID: chat.ID})}
+		arg := database.GetChatMessagesByChatIDParams{ChatID: chat.ID, AfterID: 0}
 		dbm.EXPECT().GetChatByID(gomock.Any(), chat.ID).Return(chat, nil).AnyTimes()
-		dbm.EXPECT().GetChatMessagesByChatID(gomock.Any(), chat.ID).Return(msgs, nil).AnyTimes()
-		check.Args(chat.ID).Asserts(chat, policy.ActionRead).Returns(msgs)
+		dbm.EXPECT().GetChatMessagesByChatID(gomock.Any(), arg).Return(msgs, nil).AnyTimes()
+		check.Args(arg).Asserts(chat, policy.ActionRead).Returns(msgs)
 	}))
 	s.Run("GetChatMessagesForPromptByChatID", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		chat := testutil.Fake(s.T(), faker, database.Chat{})

coderd/database/dbmetrics/querymetrics.go

@@ -1007,7 +1007,7 @@ func (m queryMetricsStore) GetChatMessageByID(ctx context.Context, id int64) (da
 	return r0, r1
 }
 
-func (m queryMetricsStore) GetChatMessagesByChatID(ctx context.Context, chatID uuid.UUID) ([]database.ChatMessage, error) {
+func (m queryMetricsStore) GetChatMessagesByChatID(ctx context.Context, chatID database.GetChatMessagesByChatIDParams) ([]database.ChatMessage, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetChatMessagesByChatID(ctx, chatID)
 	m.queryLatencies.WithLabelValues("GetChatMessagesByChatID").Observe(time.Since(start).Seconds())

coderd/database/dbmock/dbmock.go

@@ -1838,18 +1838,18 @@ func (mr *MockStoreMockRecorder) GetChatMessageByID(ctx, id any) *gomock.Call {
 }
 
 // GetChatMessagesByChatID mocks base method.
-func (m *MockStore) GetChatMessagesByChatID(ctx context.Context, chatID uuid.UUID) ([]database.ChatMessage, error) {
+func (m *MockStore) GetChatMessagesByChatID(ctx context.Context, arg database.GetChatMessagesByChatIDParams) ([]database.ChatMessage, error) {
 	m.ctrl.T.Helper()
-	ret := m.ctrl.Call(m, "GetChatMessagesByChatID", ctx, chatID)
+	ret := m.ctrl.Call(m, "GetChatMessagesByChatID", ctx, arg)
 	ret0, _ := ret[0].([]database.ChatMessage)
 	ret1, _ := ret[1].(error)
 	return ret0, ret1
 }
 
 // GetChatMessagesByChatID indicates an expected call of GetChatMessagesByChatID.
-func (mr *MockStoreMockRecorder) GetChatMessagesByChatID(ctx, chatID any) *gomock.Call {
+func (mr *MockStoreMockRecorder) GetChatMessagesByChatID(ctx, arg any) *gomock.Call {
 	mr.mock.ctrl.T.Helper()
-	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatMessagesByChatID", reflect.TypeOf((*MockStore)(nil).GetChatMessagesByChatID), ctx, chatID)
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatMessagesByChatID", reflect.TypeOf((*MockStore)(nil).GetChatMessagesByChatID), ctx, arg)
 }
 
 // GetChatMessagesForPromptByChatID mocks base method.

coderd/database/querier.go

@@ -214,7 +214,7 @@ type sqlcQuerier interface {
 	GetChatDiffStatusByChatID(ctx context.Context, chatID uuid.UUID) (ChatDiffStatus, error)
 	GetChatDiffStatusesByChatIDs(ctx context.Context, chatIds []uuid.UUID) ([]ChatDiffStatus, error)
 	GetChatMessageByID(ctx context.Context, id int64) (ChatMessage, error)
-	GetChatMessagesByChatID(ctx context.Context, chatID uuid.UUID) ([]ChatMessage, error)
+	GetChatMessagesByChatID(ctx context.Context, arg GetChatMessagesByChatIDParams) ([]ChatMessage, error)
 	GetChatMessagesForPromptByChatID(ctx context.Context, chatID uuid.UUID) ([]ChatMessage, error)
 	GetChatModelConfigByID(ctx context.Context, id uuid.UUID) (ChatModelConfig, error)
 	GetChatModelConfigByProviderAndModel(ctx context.Context, arg GetChatModelConfigByProviderAndModelParams) (ChatModelConfig, error)

coderd/database/queries.sql.go

@@ -3112,13 +3112,19 @@ FROM
     chat_messages
 WHERE
     chat_id = $1::uuid
+    AND id > $2::bigint
     AND visibility IN ('user', 'both')
 ORDER BY
     created_at ASC
 `
 
-func (q *sqlQuerier) GetChatMessagesByChatID(ctx context.Context, chatID uuid.UUID) ([]ChatMessage, error) {
-	rows, err := q.db.QueryContext(ctx, getChatMessagesByChatID, chatID)
+type GetChatMessagesByChatIDParams struct {
+	ChatID  uuid.UUID `db:"chat_id" json:"chat_id"`
+	AfterID int64     `db:"after_id" json:"after_id"`
+}
+
+func (q *sqlQuerier) GetChatMessagesByChatID(ctx context.Context, arg GetChatMessagesByChatIDParams) ([]ChatMessage, error) {
+	rows, err := q.db.QueryContext(ctx, getChatMessagesByChatID, arg.ChatID, arg.AfterID)
 	if err != nil {
 		return nil, err
 	}

coderd/database/queries/chats.sql

@@ -41,6 +41,7 @@ FROM
     chat_messages
 WHERE
     chat_id = @chat_id::uuid
+    AND id > @after_id::bigint
     AND visibility IN ('user', 'both')
 ORDER BY
     created_at ASC;