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>

lib/frame-utils.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from "vitest";
+import { frameToTime, timeToFrame } from "./frame-utils";
+
+describe("timeToFrame", () => {
+  it("round-trips every frame through frameToTime", () => {
+    for (const fps of [23.976, 24, 25, 29.97, 30]) {
+      for (let f = 0; f < 5000; f++) {
+        expect(timeToFrame(frameToTime(f, fps), fps)).toBe(f);
+      }
+    }
+  });
+
+  it("reads a seek target rounded to microseconds as that frame", () => {
+    // Browsers can report currentTime rounded, e.g. 4/24 as 0.166666
+    expect(timeToFrame(0.166666, 24)).toBe(4);
+  });
+
+  it("still floors positions inside a frame", () => {
+    expect(timeToFrame(4.5 / 24, 24)).toBe(4);
+    expect(timeToFrame(4.99 / 24, 24)).toBe(4);
+  });
+});