Add notification preferences API and screen (1n)

GET/PUT /api/v1/settings/notifications read and replace the event x
{push, chat} matrix for the five events and the reminder time. The PUT
validates every event exactly once and an HH:MM time, refuses unknown
events and fields, and writes the preferences and the household
settings' reminder_time (the one copy) in one transaction, publishing
the new settings to the scheduler as a household settings change does.

/settings/notifications renders mock 1n: the matrix with Toggle
switches, including the next-day "Keep it in rotation?" Rotation check
(D-021); the reminder time edited in a sheet; the push permission row
using pushStatus/subscribePush/unsubscribePush; the iOS InstallHint;
the Google Chat webhook card with the masked URL, "Connected · {space}"
and Send test showing success or the HTTP status; and the preview
bubble. The desktop 2e Notifications & AI tab shows the matrix and
webhook cards in its left column. The settings home row now reads Web
Push on this device and the Google Chat space, and the Household form
takes a reminder time saved elsewhere without undoing it.

Maestro task 308a460d-b22c-4179-acba-d5ea0968ccd5.

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

migrations/queries/notifications.sql

@@ -56,3 +56,13 @@ SELECT next_attempt_at FROM notification_outbox
 WHERE status = 'pending'
 ORDER BY next_attempt_at
 LIMIT 1;
+
+-- name: ListNotificationPrefs :many
+SELECT * FROM notification_prefs;
+
+-- name: UpdateNotificationPref :execrows
+-- Sets which channels an event goes to; affects no row when the event has
+-- no preference row.
+UPDATE notification_prefs
+SET push = ?, chat = ?, updated_at = ?
+WHERE event = ?;