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/settings.sql.go

@@ -161,6 +161,35 @@ func (q *Queries) GetRecipeSearchSettings(ctx context.Context) (RecipeSearchSett
 	return i, err
 }
 
+const recordChatTest = `-- name: RecordChatTest :execrows
+UPDATE chat_settings
+SET last_test_at = ?, last_test_status = ?, updated_at = ?
+WHERE id = 1 AND webhook_url_ct = ?
+`
+
+type RecordChatTestParams struct {
+	LastTestAt     sql.NullString
+	LastTestStatus sql.NullString
+	UpdatedAt      string
+	WebhookUrlCt   []byte
+}
+
+// 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.
+func (q *Queries) RecordChatTest(ctx context.Context, arg RecordChatTestParams) (int64, error) {
+	result, err := q.db.ExecContext(ctx, recordChatTest,
+		arg.LastTestAt,
+		arg.LastTestStatus,
+		arg.UpdatedAt,
+		arg.WebhookUrlCt,
+	)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected()
+}
+
 const setIntegrationsGoogleCalendar = `-- name: SetIntegrationsGoogleCalendar :execrows
 UPDATE integrations_google
 SET calendar_id = ?, calendar_name = ?, updated_at = ?