Add a schema version to persisted project and draft records

[imnasnainaec]

Persisted records (project:{id}, draft:{sourceProjectId}) have no version field. Today a format change makes old records fail type-guard validation and get skipped (projects) or reset to empty (drafts) — a silent data-loss path.

• Add modelVersion: number to InterlinearProject and DraftProject in src/types/interlinearizer.d.ts; document that consumers must treat unknown higher versions as read-only/unsupported.
• Stamp the current version (start at 1) in projectStorage.ts on every write; treat a missing field as version 0 on read.
• No migration framework yet — just the field and a single CURRENT_MODEL_VERSION constant. Migration logic can be added the first time the shape actually changes.

Size: XS–S

Priority: P1 — cheap now, expensive after #128

[imnasnainaec]

This might depend on whether we implement a CRDT backend storage.