Address Swift 6 Sendable/concurrency warnings

[james-cantonwine]

Summary

Multiple warnings related to Swift 6 strict concurrency checking need to be resolved before adopting Swift 6 language mode.

Current Warnings

PersistentModel Sendable conformance (7+ instances)

• TranscriptionService.swift — Transcript passed across actor boundaries
• AnalysisService.swift — Analysis passed across actor boundaries
• VideoAnalysisService.swift — Analysis passed across actor boundaries

These all follow the same pattern: SwiftData @Model classes are used as return values from Task blocks, crossing actor isolation boundaries. The compiler warns that PersistentModels are not Sendable.

Main actor isolation

• ServiceContainer.swift:76 — shared static property referenced from nonisolated context

Suggested Approach

• Evaluate ModelActor pattern for service layer
• Alternatively, pass persistentModelID across boundaries and re-fetch on the target actor
• Consider scoping this work to when the project is ready to adopt Swift 6 language mode

Additional (minor)

• ExportService.swift:89 — mediaBox should be let instead of var

Context

These warnings were identified during the multi-chat sessions feature implementation. They are pre-existing and unrelated to that feature.