Phase 9C.3: formalize clip prepare/export lifecycle

[FernandoAbishai]

Summary

• make Prepare an explicit gate before clip export
• invalidate prepared/failed/exported clips when output-affecting clip or timeline state changes
• make retry use the current durable draft instead of replaying stale export targets
• harden direct/batch export against stale snapshots, cancel races, and pre-job remove/edit windows
• preserve project v1 save/reopen behavior and keep backend/schema/provider unchanged

Safety invariants

• draft cannot direct/batch export
• packaged is the prepared/export-ready state
• exporting is claimed durably before the first export await
• batch re-reads each planned draft immediately before submission
• output-affecting timeline mutations demote packaged/failed/exported clips and are blocked while a clip exports
• cancel keeps busy state until polling observes a terminal job
• export folder is frozen while export is active

Validation

• all 22 frontend test:* scripts PASS
• npm run smoke:golden-creator-path PASS
• npm run lint PASS
• npm run build:frontend PASS
• git diff --check PASS
• independent blocker/high review on CURRENT tree: no blocker/high findings

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