feat: make clip batch export recoverable

[FernandoAbishai]

Summary

• make Export All use a single sequential candidate snapshot and continue after per-clip failures
• preserve completed clips, recover persisted exporting drafts as retryable failures, and retry failed drafts with current settings when no live job exists
• require confirmed non-empty output_path, show useful batch progress, and keep manifest writes best-effort

Verification

• frontend batch smoke: passed (42 assertions)
• create-clips, clip-drafts, publishing-copy, presentation, playback-sync, transcript-selection: passed
• frontend lint/build: passed
• smoke:backend: passed (93 tests)
• compileall, runtime contract, renderer policy, diff check: passed

Manual media qualification was not run because no suitable project/video fixture was available. Native candidate, public dry-run, tag, release, and merge were not run.

Base: 011ab7c
Head: bf25bc2

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