Ability to record a video for submission

As a candidate, I should be able to record and upload a video as a challenge submission.

AC:

• Add video → Record or Upload options are shown.

• Recording requests cam/mic; if denied/unsupported → fallback to Upload.

• Limits enforced: max 15 min, 500 MB, formats mp4/webm/mov → clear errors on violation.

• Upload shows progress and supports retry/resume on failure.

• After submit, video shows “Processing…” → playable preview; becomes read‑only post‑submission.

• Evaluator player: play/pause, scrub, speed 0.5×–2×, fullscreen.

• Access restricted to assigned evaluators/org members only.