Add a "Move to project" action in the video card dropdown and the
selection-mode toolbar. Videos (with their versions, comments, assets and
video-scoped share links) can be moved into another project in the same
workspace.
- New GET/POST /api/projects/[projectId]/videos/move: GET lists manageable
destination projects in the workspace; POST performs the move.
- Requires canEdit on both source and destination; same-workspace only.
- Move runs in an interactive transaction that re-asserts source ownership
atomically (updateMany guarded by projectId) to avoid a TOCTOU race, and
returns 409 on conflict. GET is rate-limited ('api').
