perf(#765): drop 6 DB roundtrips in handleTransactionUpdate

[Xuccessor]

Closes #765
Closes #763
Closes #764
Closes #766

What

Real N+1 in handleTransactionUpdate: the parties query already included user.preferences, but the loop made 6 redundant findUnique roundtrips (3 channels x 2 parties) via userPreferencesService.shouldDeliverNotification(user.id, ...).

Fix

1. Extract the channel/quiet-hours decision into a pure helper shouldDeliverNotificationFromPrefs(prefs, eventType, channel); the async method becomes a thin shim
2. Consume the already-included user.preferences in the notifications loop and call the helper 3 times locally per party — zero DB hits

Behavior parity

For users with saved prefs: identical decisions (same 4 checks). For users WITHOUT prefs: prior code triggered an upsert via findByUserId; this PR uses local schema defaults instead — the upsert side-effect is removed on this path only.

Validation

• tsc: clean
• npm test: passes (no regression)

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[nanaf6203-bit]

LGTM