Scene version selection, immutable reassembly and fourth showcase

[SubSchool]

Summary

• Expose preserved, passed scene takes with signed previews and exact render manifests.
• Let users preview old renders/takes and assemble an explicit selection into a new final version without Veo calls or repeat generation charges.
• Keep old video files, narration, caption revisions and scene reference pointers intact; recover interrupted assembly separately from scene generation.
• Add latest-by-default selectors, restore-takes-from-render action and assembly progress states.
• Host the supplied 55-second portrait video and poster as the fourth landing showcase.

Verification

• Backend tests cover selecting old takes, access/format/stale-state validation, immutable files and subtitles, no extra billing, no Veo and no automatic publication, and failed-assembly recovery.
• Ruff, web typecheck and lint passed.
• Local browser smoke: v1/v2/v3 play distinct files, old-take selection rebuilds a new final version, latest defaults return on refresh, and mobile layout fits.
• Existing customer productions were not regenerated or reassembled.