Add schema versioning and migration strategy for SwiftData

### Problem
Current SwiftData schema has NO versioning or migration strategy:
- Schema changes break existing user data
- No way to migrate from v1 → v2 safely
- Database corruption after app updates
- No rollback capability

### Proposed Solution

#### 1. Define Schema Versions
```swift
enum SchemaVersion: Int, CaseIterable {
case v1 = 1 // Initial release
case v2 = 2 // Fix Stream.source relationship with .nullify delete rule
case v3 = 3 // Add audit timestamps (createdAt, updatedAt)

static var current: SchemaVersion { .v3 }

var schema: Schema {
switch self {
case .v1: return SchemaV1.schema
case .v2: return SchemaV2.schema
case .v3: return SchemaV3.schema
}
}
}
```

#### 2. Migration Plan
```swift
enum SchemaMigrationPlan: SchemaMigrationPlan {
static var schemas: [VersionedSchema.Type] {
[SchemaV1.self, SchemaV2.self, SchemaV3.self]
}

static var stages: [MigrationStage] {
[migrateV1toV2, migrateV2toV3]
}

static let migrateV1toV2 = MigrationStage.lightweight(
fromVersion: SchemaV1.self,
toVersion: SchemaV2.self
)

static let migrateV2toV3 = MigrationStage.custom(
fromVersion: SchemaV2.self,
toVersion: SchemaV3.self,
willMigrate: { context in
let streams = try context.fetch(FetchDescriptor())
let now = Date()
for stream in streams {
stream.createdAt = now
stream.updatedAt = now
}
try context.save()
},
didMigrate: nil
)
}
```

#### 3. Update PersistenceController
```swift
container = try ModelContainer(
for: currentSchema,
migrationPlan: SchemaMigrationPlan.self,
configurations: [modelConfiguration]
)
```

### Benefits
- ✅ Safe schema evolution
- ✅ Automatic data migration
- ✅ Versioned schemas for rollback
- ✅ Custom migration logic support
- ✅ Audit trail capabilities

### Implementation Checklist
- [ ] Create `SchemaVersion.swift`
- [ ] Create `SchemaV1.swift` (current production)
- [ ] Create `SchemaV2.swift` (with relationship fixes)
- [ ] Create `SchemaV3.swift` (with timestamps)
- [ ] Create `SchemaMigrationPlan.swift`
- [ ] Update `PersistenceController`
- [ ] Add migration tests
- [ ] Document migration process

### Files to Create
- `SchemaVersion.swift`
- `SchemaV1.swift`
- `SchemaV2.swift`
- `SchemaV3.swift`
- `SchemaMigrationPlan.swift`

Start with PersistenceController and the existing SwiftData model definitions, then review the proposed SchemaVersion.swift, SchemaV1.swift, SchemaV2.swift, SchemaV3.swift, and SchemaMigrationPlan.swift files. Add migration tests and document the migration process; done means versioned schemas, the listed migration stages, updated container setup, and safe handling of existing data.

Written by the indexing model from the issue text.