Implement ModelActor for thread-safe SwiftData access

### Problem
Current SwiftData access pattern has concurrency safety issues:
- Models marked `@unchecked Sendable` without proper isolation
- Direct `ModelContext` access from multiple actors
- No structured concurrency boundaries
- Risk of data races and crashes

### Proposed Solution: ModelActor Pattern

#### Create ArkavoDataActor
```swift
@ModelActor
actor ArkavoDataActor {
func getOrCreateAccount() throws -> Account {
let descriptor = FetchDescriptor(predicate: #Predicate { $0.id == 0 })
if let account = try modelContext.fetch(descriptor).first {
return account
}
let account = Account()
modelContext.insert(account)
try modelContext.save()
return account
}

func fetchProfile(withPublicID publicID: Data) throws -> Profile? {
let descriptor = FetchDescriptor(
predicate: #Predicate { $0.publicID == publicID }
)
return try modelContext.fetch(descriptor).first
}

func saveThought(_ thought: Thought) throws -> Bool {
let existing = try modelContext.fetch(
FetchDescriptor(predicate: #Predicate { $0.publicID == thought.publicID })
)
guard existing.isEmpty else { return false }

modelContext.insert(thought)
try modelContext.save()
return true
}
}
```

#### Usage Pattern
```swift
class CreatorViewModel: ObservableObject {
@Published var videoThoughts: [Thought] = []
private let dataActor: ArkavoDataActor

init(container: ModelContainer) {
self.dataActor = ArkavoDataActor(modelContainer: container)
}

func loadVideoThoughts() async {
do {
let thoughts = try await dataActor.fetchThoughts(
forStreamPublicID: videoStreamPublicID
)
await MainActor.run {
self.videoThoughts = thoughts
}
} catch {
print("Error: \(error)")
}
}
}
```

### Benefits
- ✅ Structured concurrency with clear actor boundaries
- ✅ No `@unchecked Sendable` needed
- ✅ Compile-time actor isolation checking
- ✅ Prevents data races
- ✅ Centralized data access logic

### Implementation Checklist
- [ ] Create `ArkavoDataActor.swift` with `@ModelActor`
- [ ] Migrate database operations to actor methods
- [ ] Update `PersistenceController` to provide actor instances
- [ ] Update ViewModels to use actor pattern
- [ ] Remove `@unchecked Sendable` annotations
- [ ] Add concurrency tests

### Files to Create
- `ArkavoDataActor.swift`

### Files to Modify
- `PersistenceController.swift`
- `CreatorViewModel.swift`
- `VideoFeedViewModel.swift`
- `ChatViewModel.swift`
- `GroupViewModel.swift`

Start by reading PersistenceController.swift and the proposed ArkavoDataActor.swift structure, then review CreatorViewModel.swift, VideoFeedViewModel.swift, ChatViewModel.swift, and GroupViewModel.swift. Trace existing database operations and @unchecked Sendable annotations before planning the migration. Done means actor-based access is adopted, the listed view models are updated, unsafe annotations are removed, and concurrency tests are added.

Written by the indexing model from the issue text.