Allow multiple push notification subscriptions per user (multi-device support)

[hrmtc]

Summary

Push notifications are back in 1.5.6, but each user only ends up with a single active subscription. Registering a second device overwrites the first one, so the earlier device stops receiving pushes.

Steps to Reproduce

1. Run Split Pro 1.5.6 (e.g. ossapps/splitpro:1.5.6).
2. Sign in as the same user on browser/device A and opt in to notifications.
3. Sign in as the same user on browser/device B and enable notifications there as well.
4. Trigger a push (settle an expense, add a shared expense, etc.).

Current Behavior

Only the most recently registered device receives the notification. Inspecting the PushNotification table (prisma/schema.prisma:248) shows that the new subscription replaces the previous row, so the first endpoint disappears. This matches the current API flow in src/server/api/routers/user.ts:178, which effectively leaves us with one row per user.

Expected Behavior

Every device that successfully subscribes should keep its own push subscription so all of them receive notifications.

Proposed Changes

• Remove/adjust the unique constraint that limits PushNotification to a single row per userId, and add the migration to drop it in production.
• Update the updatePushNotification mutation so creating a new subscription never evicts the existing ones; keep the cleanup path on explicit unsubscribe.
• Double-check sendExpensePushNotification still fans out to every subscription (it already uses findMany, so just make sure nothing else filters).
• Add regression coverage (API or integration) proving that registering a second device preserves the first subscription.

Additional Context

This came up while verifying push notifications on 1.5.6 in #445.

[krokosik]

Interesting point about regression coverage. Are you familiar with ways to test web push?