feat(coderd): add new dispatch logic for coder inbox (#16764)

This PR is [resolving the dispatch part of Coder
Inbocx](https://github.com/coder/internal/issues/403).

Since the DB layer has been merged - we now want to insert notifications
into Coder Inbox in parallel of the other delivery target.

To do so, we push two messages instead of one using the `Enqueue`
method.

coderd/notifications/dispatch/inbox.go

@@ -0,0 +1,81 @@
+package dispatch
+
+import (
+	"context"
+	"encoding/json"
+	"text/template"
+
+	"golang.org/x/xerrors"
+
+	"cdr.dev/slog"
+
+	"github.com/google/uuid"
+
+	"github.com/coder/coder/v2/coderd/database"
+	"github.com/coder/coder/v2/coderd/database/dbtime"
+	"github.com/coder/coder/v2/coderd/notifications/types"
+	markdown "github.com/coder/coder/v2/coderd/render"
+)
+
+type InboxStore interface {
+	InsertInboxNotification(ctx context.Context, arg database.InsertInboxNotificationParams) (database.InboxNotification, error)
+}
+
+// InboxHandler is responsible for dispatching notification messages to the Coder Inbox.
+type InboxHandler struct {
+	log   slog.Logger
+	store InboxStore
+}
+
+func NewInboxHandler(log slog.Logger, store InboxStore) *InboxHandler {
+	return &InboxHandler{log: log, store: store}
+}
+
+func (s *InboxHandler) Dispatcher(payload types.MessagePayload, titleTmpl, bodyTmpl string, _ template.FuncMap) (DeliveryFunc, error) {
+	subject, err := markdown.PlaintextFromMarkdown(titleTmpl)
+	if err != nil {
+		return nil, xerrors.Errorf("render subject: %w", err)
+	}
+
+	htmlBody, err := markdown.PlaintextFromMarkdown(bodyTmpl)
+	if err != nil {
+		return nil, xerrors.Errorf("render html body: %w", err)
+	}
+
+	return s.dispatch(payload, subject, htmlBody), nil
+}
+
+func (s *InboxHandler) dispatch(payload types.MessagePayload, title, body string) DeliveryFunc {
+	return func(ctx context.Context, msgID uuid.UUID) (bool, error) {
+		userID, err := uuid.Parse(payload.UserID)
+		if err != nil {
+			return false, xerrors.Errorf("parse user ID: %w", err)
+		}
+		templateID, err := uuid.Parse(payload.NotificationTemplateID)
+		if err != nil {
+			return false, xerrors.Errorf("parse template ID: %w", err)
+		}
+
+		actions, err := json.Marshal(payload.Actions)
+		if err != nil {
+			return false, xerrors.Errorf("marshal actions: %w", err)
+		}
+
+		// nolint:exhaustruct
+		_, err = s.store.InsertInboxNotification(ctx, database.InsertInboxNotificationParams{
+			ID:         msgID,
+			UserID:     userID,
+			TemplateID: templateID,
+			Targets:    payload.Targets,
+			Title:      title,
+			Content:    body,
+			Actions:    actions,
+			CreatedAt:  dbtime.Now(),
+		})
+		if err != nil {
+			return false, xerrors.Errorf("insert inbox notification: %w", err)
+		}
+
+		return false, nil
+	}
+}