feat: add chat cost analytics backend (#23036)

Add cost tracking for LLM chat interactions with microdollar precision.

## Changes
- Add `chatcost` package for per-message cost calculation using
`shopspring/decimal` for intermediate arithmetic
- **Ceil rounding policy**: fractional micros round UP to next whole
micro (applied once after summing all components)
- Database migration: `total_cost_micros` BIGINT column with historical
backfill and `created_at` index
- API endpoints: per-user cost summary and admin rollup under
`/api/experimental/chats/cost/`
- SDK types: `ChatCostSummary`, `ChatCostModelBreakdown`,
`ChatCostUserRollup`
- Fix `modeloptionsgen` to handle `decimal.Decimal` as opaque numeric
type
- Update frontend pricing test fixtures for string decimal types

## Design decisions
- `NULL` = unpriced (no matching model config), `0` = free
- Reasoning tokens included in output tokens (no double-counting)
- Integer microdollars (BIGINT) for storage and API responses
- Price config uses `decimal.Decimal` for exact parsing; totals use
`int64`

Frontend: #23037

coderd/database/dbauthz/dbauthz.go

@@ -2426,6 +2426,34 @@ func (q *querier) GetChatByIDForUpdate(ctx context.Context, id uuid.UUID) (datab
 	return fetch(q.log, q.auth, q.db.GetChatByIDForUpdate)(ctx, id)
 }
 
+func (q *querier) GetChatCostPerChat(ctx context.Context, arg database.GetChatCostPerChatParams) ([]database.GetChatCostPerChatRow, error) {
+	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceChat.WithOwner(arg.OwnerID.String())); err != nil {
+		return nil, err
+	}
+	return q.db.GetChatCostPerChat(ctx, arg)
+}
+
+func (q *querier) GetChatCostPerModel(ctx context.Context, arg database.GetChatCostPerModelParams) ([]database.GetChatCostPerModelRow, error) {
+	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceChat.WithOwner(arg.OwnerID.String())); err != nil {
+		return nil, err
+	}
+	return q.db.GetChatCostPerModel(ctx, arg)
+}
+
+func (q *querier) GetChatCostPerUser(ctx context.Context, arg database.GetChatCostPerUserParams) ([]database.GetChatCostPerUserRow, error) {
+	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceChat); err != nil {
+		return nil, err
+	}
+	return q.db.GetChatCostPerUser(ctx, arg)
+}
+
+func (q *querier) GetChatCostSummary(ctx context.Context, arg database.GetChatCostSummaryParams) (database.GetChatCostSummaryRow, error) {
+	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceChat.WithOwner(arg.OwnerID.String())); err != nil {
+		return database.GetChatCostSummaryRow{}, err
+	}
+	return q.db.GetChatCostSummary(ctx, arg)
+}
+
 func (q *querier) GetChatDiffStatusByChatID(ctx context.Context, chatID uuid.UUID) (database.ChatDiffStatus, error) {
 	// Authorize read on the parent chat.
 	_, err := q.GetChatByID(ctx, chatID)

coderd/database/dbauthz/dbauthz_test.go

@@ -438,6 +438,81 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().GetChatByIDForUpdate(gomock.Any(), chat.ID).Return(chat, nil).AnyTimes()
 		check.Args(chat.ID).Asserts(chat, policy.ActionRead).Returns(chat)
 	}))
+	s.Run("GetChatCostPerChat", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		arg := database.GetChatCostPerChatParams{
+			OwnerID:   uuid.New(),
+			StartDate: time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC),
+			EndDate:   time.Date(2025, 2, 1, 0, 0, 0, 0, time.UTC),
+		}
+		rows := []database.GetChatCostPerChatRow{{
+			RootChatID:        uuid.New(),
+			ChatTitle:         "chat-cost",
+			TotalCostMicros:   123,
+			MessageCount:      4,
+			TotalInputTokens:  55,
+			TotalOutputTokens: 89,
+		}}
+		dbm.EXPECT().GetChatCostPerChat(gomock.Any(), arg).Return(rows, nil).AnyTimes()
+		check.Args(arg).Asserts(rbac.ResourceChat.WithOwner(arg.OwnerID.String()), policy.ActionRead).Returns(rows)
+	}))
+	s.Run("GetChatCostPerModel", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		arg := database.GetChatCostPerModelParams{
+			OwnerID:   uuid.New(),
+			StartDate: time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC),
+			EndDate:   time.Date(2025, 2, 1, 0, 0, 0, 0, time.UTC),
+		}
+		rows := []database.GetChatCostPerModelRow{{
+			ModelConfigID:     uuid.New(),
+			DisplayName:       "GPT 4.1",
+			Provider:          "openai",
+			Model:             "gpt-4.1",
+			TotalCostMicros:   456,
+			MessageCount:      7,
+			TotalInputTokens:  144,
+			TotalOutputTokens: 233,
+		}}
+		dbm.EXPECT().GetChatCostPerModel(gomock.Any(), arg).Return(rows, nil).AnyTimes()
+		check.Args(arg).Asserts(rbac.ResourceChat.WithOwner(arg.OwnerID.String()), policy.ActionRead).Returns(rows)
+	}))
+	s.Run("GetChatCostPerUser", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		arg := database.GetChatCostPerUserParams{
+			PageOffset: 0,
+			PageLimit:  25,
+			StartDate:  time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC),
+			EndDate:    time.Date(2025, 2, 1, 0, 0, 0, 0, time.UTC),
+			Username:   "cost-user",
+		}
+		rows := []database.GetChatCostPerUserRow{{
+			UserID:            uuid.New(),
+			Username:          "cost-user",
+			Name:              "Cost User",
+			AvatarURL:         "https://example.com/avatar.png",
+			TotalCostMicros:   789,
+			MessageCount:      11,
+			ChatCount:         3,
+			TotalInputTokens:  377,
+			TotalOutputTokens: 610,
+			TotalCount:        1,
+		}}
+		dbm.EXPECT().GetChatCostPerUser(gomock.Any(), arg).Return(rows, nil).AnyTimes()
+		check.Args(arg).Asserts(rbac.ResourceChat, policy.ActionRead).Returns(rows)
+	}))
+	s.Run("GetChatCostSummary", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		arg := database.GetChatCostSummaryParams{
+			OwnerID:   uuid.New(),
+			StartDate: time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC),
+			EndDate:   time.Date(2025, 2, 1, 0, 0, 0, 0, time.UTC),
+		}
+		row := database.GetChatCostSummaryRow{
+			TotalCostMicros:      987,
+			PricedMessageCount:   12,
+			UnpricedMessageCount: 2,
+			TotalInputTokens:     400,
+			TotalOutputTokens:    800,
+		}
+		dbm.EXPECT().GetChatCostSummary(gomock.Any(), arg).Return(row, nil).AnyTimes()
+		check.Args(arg).Asserts(rbac.ResourceChat.WithOwner(arg.OwnerID.String()), policy.ActionRead).Returns(row)
+	}))
 	s.Run("GetChatDiffStatusByChatID", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		chat := testutil.Fake(s.T(), faker, database.Chat{})
 		diffStatus := testutil.Fake(s.T(), faker, database.ChatDiffStatus{ChatID: chat.ID})