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>

migrations/queries/settings.sql

@@ -146,3 +146,11 @@ ON CONFLICT (id) DO UPDATE SET
 	last_test_status = excluded.last_test_status,
 	updated_at = excluded.updated_at
 RETURNING *;
+
+-- name: RecordChatTest :execrows
+-- 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.
+UPDATE chat_settings
+SET last_test_at = ?, last_test_status = ?, updated_at = ?
+WHERE id = 1 AND webhook_url_ct = ?;