Add Google Chat webhook sender and settings API (Feature 19)

internal/chat keeps the incoming webhook in chat_settings, sealed with
secretbox, and is the notify sender of the chat channel.

- PUT /api/v1/settings/chat-webhook {url, space_label} accepts only
  https://chat.googleapis.com/v1/spaces/{space}/messages?key=…&token=…;
  GET answers the space label, a masked URL tail and the last test outcome.
- POST /api/v1/settings/chat-webhook/test posts "MealQuest · test message",
  answers Google Chat's HTTP status and records last_test_at/status.
- Notifications post {text}, plus cardsV2 with an "Open" button under
  MEALQUEST_PUBLIC_URL for plan_ready and list_ready, threaded by
  threadKey=mealquest-week-YYYY-Www with
  REPLY_MESSAGE_FALLBACK_TO_NEW_THREAD. 429 and 5xx are retried; any other
  non-2xx fails the row at once through the new notify.ErrUndeliverable.
- One place builds every event's title, body and Chat text.

The webhook URL never appears in a response, log line or error; tests
against an httptest Chat endpoint scan both.

Maestro task 919e9422-3a41-4f1f-8967-77da17f5f9f3.

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

internal/server/google_test.go

@@ -68,7 +68,7 @@ func newGoogleEnv(t *testing.T) *googleEnv {
 		t.Fatal(err)
 	}
 	logger := slog.New(slog.NewJSONHandler(e.logs, &slog.HandlerOptions{Level: slog.LevelDebug}))
-	e.h, err = NewRouter(logger, "dev", nil, stubAuth{}, stubSessions{}, newTestSettings(t), newTestSearch(t), newTestAI(t),
+	e.h, err = NewRouter(logger, "dev", nil, stubAuth{}, stubSessions{}, newTestSettings(t), newTestSearch(t), newTestAI(t), newTestChat(t),
 		newTestDinners(t), newTestSuggest(t), newTestPlan(t), newTestShopping(t), svc, newTestPush(t), newTestRunner(t), newTestPhotos(t),
 		fstest.MapFS{"index.html": {Data: []byte(fixtureIndex)}})
 	if err != nil {