feat(review): flag handles in the review player (metadata only)

Renders run 8 frames past the cut at each end. Directors were flagging
glitches in unfixed handles as shot problems, so the player now marks
them — without trimming or burning anything into the EXRs/MOV/MP4.

- Version.handleHead/handleTail/pictureStartFrame, Export.handleHead/
  handleTail; migration backfills pipeline exports and their preview
  versions from the manifest's handleFrames (else 8)
- AE panel: "Includes handles [8] frames each end" -> manifest.review;
  older manifests fall back to source.handleFrames
- finalize copies handles onto the Version; worker 1.6.4 reports the
  head slate length (pictureStartSeconds), else the player infers it
  as MP4 frames - EXR frameCount
- Player: hatched HANDLE / SLATE regions and cut marks on the timeline,
  "HEAD/TAIL HANDLE · NOT IN EDIT" overlay on handle frames, HANDLE badge
  on notes, "Edit range only" toggle (default on in the client portal)
- Staff Handles editor on the review page for manual uploads/corrections

Also fixes two player bugs found while testing: timeToFrame floored
float error (every seek landed a frame early), and J/reverse never ran
(the keyboard effect's cleanup stopped it on the next render).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

RenderWorker/VFXReviewWorker/JobExecutor.cs

@@ -461,6 +461,9 @@ public sealed class JobExecutor
                     ["frameCount"] = result.Build?.FrameCount,
                     ["fps"] = manifest.Fps,
                     ["fileName"] = Path.GetFileName(result.Mp4Path!),
+                    // Slate length, so the review player can place the handle
+                    // markers exactly instead of inferring it (server ≥ 2026-09-22).
+                    ["pictureStartSeconds"] = result.Build?.PictureStartSeconds,
                 },
             });