feat: first-run demo mode (no OAuth required)

[jdwit]

Summary

• New ytstudio demo sub-app: videos, analytics, comments, tour, info run the real command code paths against a built-in fake channel with no network calls.
• YTSTUDIO_DEMO=1 ytstudio <any-command> routes get_data_service and get_analytics_service to the fake services, so any existing command works in demo mode out of the box.
• README and docs gain a "Try in 10 seconds" block (pipx install ytstudio-cli then ytstudio demo tour) that drops the OAuth gate from the first-run experience.
• scripts/terminaltrove-demo.tape rewritten to record the demo tour, so the terminaltrove GIF no longer needs a sandbox channel.

Highlights

• ytstudio demo tour [--no-pauses]: end-to-end run of videos -> analytics -> comments with deterministic pacing for VHS.
• YTSTUDIO_DEMO=1 ytstudio videos list (and any other read command) works against bundled JSON fixtures shipped in ytstudio/fixtures/.
• ytstudio status shows mode: demo and short-circuits the credentials check when the env var is set.
• One-shot banner per process on stderr (demo mode: using built-in fake channel (no network)), suppressed for -o json so piped JSON stays clean.
• Fake analytics report falls back to a stable, hash-seeded synthetic response for non-overview metric combinations.

Test plan

• uv sync --group dev
• uv run ruff check .
• uv run ruff format --check .
• uv run pytest -q (234 passed, including 19 new tests; demo + demo_service coverage >= 90%)
• uv build confirms ytstudio/fixtures/*.json ships in the wheel
• Smoke test: pipx install . && ytstudio demo tour --no-pauses
• Re-record terminaltrove GIF with vhs scripts/terminaltrove-demo.tape

Out of scope

• No record/replay against a real channel (fixtures are fully synthetic).
• Tape file is updated but the GIF re-record itself is a manual step.

[jdwit]

Closing per current direction; only the playlists PR (#25) is being shipped from this trio. Branch left in place in case we revisit.