docs(demo): captioned demo media + README gif (P4.7)

[franciszver]

Closes #58

Summary

Adds a captioned demo of the live app to the README, plus a reusable build pipeline.

• docs/assets/demo.gif (tracked, 706 KB, embedded near the top of the README) — 7 captioned screens of the live demo, 3s each, looping.
• .demo-media/demo.mp4 (gitignored, delivered separately) — same frames with a gentle stdlib-synthesized soundtrack; confirmed AAC audio stream.
• scripts/build_demo_media.py — PNG frames → captioned gif + mp4. Pillow for captions/gif; bundled ffmpeg via imageio-ffmpeg for the mp4; music synthesized with stdlib wave+math (no numpy). --no-music toggle.
• scripts/test_build_demo_media.py — hermetic pytest tests, importorskip-gated (skip cleanly where Pillow/ffmpeg absent): caption changes the frame, music-on→audio / off→none, odd dimensions don't crash. 4 passed.

The 7 screens

login → dashboard → editor → clause library → jurisdiction/AI-context → AI suggestions → admin audit trail, captions in .demo-media/captions.json.

Notes for review

• Slot 5 substitution: there is no reachable Bluebook-citations UI in the running app — CitationBrowser/CitationManager are orphaned (not imported by any page), so the capture used the editor's jurisdiction/AI-context panel instead ("AI-tailored to your jurisdiction"). Worth deciding later whether to wire those components in or drop the citation feature from the marketing copy.
• Verified by eye: two captioned frames opened and confirmed legible; gif = 7 frames; mp4 has an audio stream.
• CI here is JS-only, so the Python pipeline tests run locally (they were run: 4 passed), not in CI.
• Left open for owner review because it changes the public README (per the public-facing-text approval rule). The mp4 is delivered as a file, not committed.

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