Generate Segment Assets Endpoint

Generate a project's segment assets — images, clips, overlays and fetched B-roll / real photos.

With no targets it fills gaps: every asset not yet generated (or stale, or failed) and not already generating, within asset_scope, outside segments being rewritten. Named targets — assets (segment asset ids, each with optional model / settings / editable_sections, saved to the asset's prompt editor) or segment_numbers — run exactly those, re-rendering finished ones. A real photo placed over a segment's image re-fetches only when its asset id is named. dry_run returns the count and credit estimate without queueing.