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/db/querier.go

@@ -260,6 +260,10 @@ type Querier interface {
 	// event id while its dinner stays the same; a new dinner clears it, since
 	// Calendar no longer shows the day as planned.
 	PutPlanDay(ctx context.Context, arg PutPlanDayParams) error
+	// Records the outcome of a test message sent to the webhook sealed in
+	// webhook_url_ct; affects no row when the webhook has been replaced or
+	// removed since the test began.
+	RecordChatTest(ctx context.Context, arg RecordChatTestParams) (int64, error)
 	// Counts a delivery to a subscription that the push service did not accept.
 	RecordPushFailure(ctx context.Context, id string) error
 	// Records that the push service accepted a notification for a subscription.