iCloud sync (CloudKit-backed SwiftData)

[ryanabooth]

Flip on CloudKit-backed SwiftData so users can sync across devices.

Scope

Schema is already CloudKit-compatible — every property has a default, every relationship is optional with an explicit inverse, no @Attribute(.unique). The remaining work:

1. Enable iCloud capability + CloudKit container in Xcode (paid Apple Developer account required)
2. Update LifePlanner/Repositories/Database/ModelContainer.swift to use ModelConfiguration(cloudKitDatabase:)
3. Test on two devices signed into the same Apple ID

Blocker

Paid Apple Developer account. Currently shipping via free TestFlight which does not include CloudKit container provisioning.

Acceptance criteria (post-unblock)

• iCloud capability + container enabled in app target
• ModelConfiguration uses .private(databaseName:)
• Adding a goal on iPhone A appears on iPhone B within 60s (both signed into same Apple ID)
• Account-switch scenario doesn't corrupt local state
• Conflict resolution tested (offline edits on two devices, then both come online)