Mark each scene good or to-work-on, and filter the edit by it

[pizzato]

Closes #383.

On a film with many scenes there was nothing to say which scenes had been
looked at and which were still being worked on.

What changed

Three review categories. Every scene on Edit film → Scenes sits in one of
them, picked with a three-way control on its own card:

Category	Means
To be reviewed	Nobody has looked at it yet — where every scene starts
To work on	Looked at, and it still needs work
Approved	Signed off, leave it alone

The category also shows as a chip on the collapsed card, beside the
Rendered/No video chip.

A filter for the pass. Above the cards: All (n) plus one segment per
category with its count, each showing exactly its own scenes. The line beside
it counts how many are approved — "Every scene is approved — this film is
fully edited." once they all are.

Filtering only hides cards. Scene numbering, the move chevrons, Reassemble and
Add scene all still act on the whole film.

Storage. Marks live in scene_review.json beside the film — a note about
the edit, not part of what renders, so nothing in the render path reads it. An
empty mark is to-be-reviewed, so the default category costs no state. Marks
ride out on GET /api/films/scenes as review, are written by the new POST /api/films/scenes/{scene_id}/review, and are dropped when their scene is
deleted.

Verification

• tests/test_scene_review.py — marks persist and come back on the scene list,
  empty status clears, unknown status rejected, delete drops the mark, an
  unreadable store is ignored, path escape rejected.
• Full suite: 1910 passed. ruff clean. npm run build clean.
• Driven live against the real catalogue on a preview server (background loops
  off): marks round-trip to the sidecar and back through GET /api/films/scenes, "good" is rejected as an unknown status, set/clear both
  work.

Docs: new Review marks section in docs/manual/edit-film.md, plus the
README feature line.

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