feat(api): add FCM_ENDPOINT transport override for integration tests

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: AchoArnold

api/pkg/di/container.go

@@ -5,6 +5,7 @@ import (
 	"crypto/tls"
 	"fmt"
 	"net/http"
+	"net/url"
 	"os"
 	"strconv"
 	"strings"
@@ -398,7 +399,26 @@ ALTER TABLE discords ADD CONSTRAINT IF NOT EXISTS uni_discords_server_id CHECK (
 // FirebaseApp creates a new instance of firebase.App
 func (container *Container) FirebaseApp() (app *firebase.App) {
 	container.logger.Debug(fmt.Sprintf("creating %T", app))
-	app, err := firebase.NewApp(context.Background(), nil, option.WithAuthCredentialsJSON(option.ServiceAccount, container.FirebaseCredentials()))
+
+	var opts []option.ClientOption
+
+	if fcmEndpoint := os.Getenv("FCM_ENDPOINT"); fcmEndpoint != "" {
+		container.logger.Info(fmt.Sprintf("using FCM endpoint override: %s", fcmEndpoint))
+		targetURL, err := url.Parse(fcmEndpoint)
+		if err != nil {
+			container.logger.Fatal(stacktrace.Propagate(err, "cannot parse FCM_ENDPOINT"))
+		}
+		opts = append(opts, option.WithHTTPClient(&http.Client{
+			Transport: &fcmRedirectTransport{
+				target: targetURL,
+				base:   http.DefaultTransport,
+			},
+		}))
+	} else {
+		opts = append(opts, option.WithAuthCredentialsJSON(option.ServiceAccount, container.FirebaseCredentials()))
+	}
+
+	app, err := firebase.NewApp(context.Background(), nil, opts...)
 	if err != nil {
 		msg := "cannot initialize firebase application"
 		container.logger.Fatal(stacktrace.Propagate(err, msg))

api/pkg/di/fcm_transport.go

@@ -0,0 +1,19 @@
+package di
+
+import (
+	"net/http"
+	"net/url"
+)
+
+// fcmRedirectTransport rewrites Firebase SDK HTTP requests to a custom endpoint.
+// Used in integration tests to redirect FCM traffic to the emulator.
+type fcmRedirectTransport struct {
+	target *url.URL
+	base   http.RoundTripper
+}
+
+func (t *fcmRedirectTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	req.URL.Scheme = t.target.Scheme
+	req.URL.Host = t.target.Host
+	return t.base.RoundTrip(req)
+}