Turn on push from onboarding step 3 and add a test-push endpoint (Feature 19)

Onboarding step 3's "Turn on notifications" now calls usePush().subscribePush
straight from that tap (the only place onboarding asks for permission) and
shows how it ended: on; blocked, with how to allow it in the browser's site
settings; not supported, with how to add MealQuest to the Home Screen on
iPhone; or a dismissed prompt that can be asked again. "Maybe later" still
finishes without asking. The interim notificationsUnavailable is gone.

POST /api/v1/settings/notifications/test-push queues "MealQuest test
notification" through the notification outbox as a plan_ready event with a
fresh plan_ready:test-<ULID> key, opening /plan, on the push channel only
(new notify.Service.EmitPush), never Google Chat. It answers 202
{event_key, subscriptions}, or 409 no_subscriptions / push_off rather than
reporting success when nothing would be sent.

Maestro task f89279b8-b92c-4d9f-a6c2-fd87fccda509.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/push/push.go

@@ -14,6 +14,7 @@ import (
 	"strings"
 
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/db"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/notify"
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/store"
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/vapid"
 )
@@ -69,22 +70,75 @@ func (e *ValidationError) Error() string {
 	return "invalid push subscription: " + strings.Join(parts, "; ")
 }
 
-// Service stores push subscriptions.
+// Service stores push subscriptions and sends a test notification to them.
 type Service struct {
 	st        *store.Store
 	publicKey string
+	notifier  *notify.Service
 }
 
 // NewService returns a Service over st handing out publicKey, the VAPID
-// public key in vapid's encoding.
-func NewService(st *store.Store, publicKey string) (*Service, error) {
+// public key in vapid's encoding, that sends its test notification through
+// notifier.
+func NewService(st *store.Store, publicKey string, notifier *notify.Service) (*Service, error) {
 	if st == nil {
 		return nil, errors.New("push: nil store")
 	}
 	if _, err := vapid.ParsePublicKey(publicKey); err != nil {
 		return nil, fmt.Errorf("push: VAPID public key: %w", err)
 	}
-	return &Service{st: st, publicKey: publicKey}, nil
+	if notifier == nil {
+		return nil, errors.New("push: nil notifier")
+	}
+	return &Service{st: st, publicKey: publicKey, notifier: notifier}, nil
+}
+
+// The test notification SendTest sends.
+const (
+	TestTitle = "MealQuest test notification"
+	TestBody  = "Push works on this device. Tap to open the plan."
+	TestURL   = "/plan"
+)
+
+// ErrNoSubscriptions is returned by SendTest when no device has turned on
+// push: there is nothing to send the test to.
+var ErrNoSubscriptions = errors.New("push: no device has turned on notifications")
+
+// TestSent is a test notification queued for delivery: its event key and
+// how many subscriptions were stored when it was queued.
+type TestSent struct {
+	EventKey      string `json:"event_key"`
+	Subscriptions int    `json:"subscriptions"`
+}
+
+// SendTest queues TestTitle for every stored subscription, through the
+// notification outbox as a plan_ready event whose tap opens TestURL. Its
+// key, "plan_ready:test-" and a new id, is never used twice, so it is never
+// taken for an event already sent. It goes to push alone, never to Google
+// Chat. With no subscription it is ErrNoSubscriptions, and with push
+// switched off for plan_ready it is notify.ErrPushOff; neither queues
+// anything. Delivery happens after SendTest returns, with the outbox's
+// retries.
+func (s *Service) SendTest(ctx context.Context) (TestSent, error) {
+	subs, err := db.New(s.st.Reader).ListPushSubscriptions(ctx)
+	if err != nil {
+		return TestSent{}, fmt.Errorf("push: list subscriptions: %w", err)
+	}
+	if len(subs) == 0 {
+		return TestSent{}, ErrNoSubscriptions
+	}
+	key := string(notify.PlanReady) + ":test-" + store.NewID()
+	err = s.notifier.EmitPush(ctx, notify.Event{
+		Type:  notify.PlanReady,
+		Key:   key,
+		Title: TestTitle,
+		Body:  TestBody,
+		URL:   TestURL,
+	})
+	if err != nil {
+		return TestSent{}, fmt.Errorf("push: send test notification: %w", err)
+	}
+	return TestSent{EventKey: key, Subscriptions: len(subs)}, nil
 }
 
 // PublicKey returns the VAPID public key, for applicationServerKey.