feat: livestreams command for managing YouTube live broadcasts

[jdwit]

Summary

Add a new ytstudio livestreams typer app with list, show, schedule, start, stop, and update subcommands driving the YouTube liveBroadcasts API. Inspired by ParkerClelland/ytstudio-cli but rewritten against the current codebase and reviewed against the YouTube Live API docs.

What you get

• livestreams list with --status all|upcoming|active|completed and table/json output.
• livestreams show <id> with optional --ingest (RTMP/RTMPS URLs from the bound liveStream) and --show-key (reveals the stream key; redacted by default).
• livestreams schedule with --title, --scheduled-start, optional --scheduled-end, --privacy, --description, --made-for-kids/--not-made-for-kids and --execute (dry-run is default).
• livestreams start <id> with --to testing|live for the monitor-stream-first workflow.
• livestreams stop <id> for transition to complete.
• livestreams update <id> partial update; only sends the parts that actually changed, but always carries contentDetails.monitorStream when contentDetails is touched (YouTube rejects updates that omit it).

Improvements vs. the fork

• Stream key plumbing. show --ingest fetches the bound liveStream and prints ingestionAddress/rtmpsIngestionAddress plus their backups; streamName is masked unless --show-key is passed explicitly.
• Monitor-stream preservation. Update keeps monitorStream.enableMonitorStream and broadcastStreamDelayMs on every contentDetails write, fixing the fork's silent bug.
• update aligned with API surface. Drops the --made-for-kids flag here because liveBroadcasts.update only accepts privacyStatus under status; the COPPA flag is set at schedule time.
• Friendly errors. livePermissionBlocked, insufficientLivePermissions, errorExecutingTransition, rate-limit and concurrency errors map to actionable messages instead of raw HttpError.
• Typed output. --output is an enum (table/json); anything else is rejected.
• No em-dash / smart unicode. Uses ASCII - for empty cells.
• Demo mode. _DemoLiveBroadcasts + _DemoLiveStreams and broadcasts.json / live_streams.json fixtures so YTSTUDIO_DEMO=1 drives the new commands offline.

Out of scope (deliberate follow-ups)

• Auto-create-and-bind a liveStream from schedule (today you bind in Studio or with an encoder). Would add ~50 quota units and another OAuth-scoped write; worth a separate PR.
• Setting made-for-kids on the broadcast's resulting video resource via videos.update.

Review

Reviewed first by myself, then handed to Codex via the rescue subagent. Codex returned 1 high (the monitorStream bug), 2 medium (status update surface, demo update validation), 3 low (missing error reasons, missing RTMPS backup, untyped --output). All five were applied in commit b1a6e54; the demo update validation was intentionally left as echo since demo mode is an offline aid, not a contract test.

Test plan

• pytest -q -> 164 passed (was 134 on main)
• ruff check . and ruff format --check . -> clean
• YTSTUDIO_DEMO=1 ytstudio livestreams list --status all shows seeded demo broadcasts
• YTSTUDIO_DEMO=1 ytstudio livestreams show demo-broadcast-upcoming-1 --ingest shows redacted stream key
• YTSTUDIO_DEMO=1 ytstudio livestreams update demo-broadcast-upcoming-1 --no-dvr dry-run shows old/new diff
• Smoke test against a real channel with live streaming enabled (manual, before/after merge)