feat(app-store): add BigBlueButton video conferencing integration

[kejuunuy]

Summary

Adds a BigBlueButton conferencing app to the app-store, addressing the
long-standing request in #1985 / CAL-3105.

The integration follows the existing jitsivideo pattern (dynamic location,
shared-secret auth — no OAuth) and exposes a real VideoApiAdapter that
provisions meetings on the BBB server:

• New app at packages/app-store/bigbluebutton/ with _metadata.ts,
  package.json, index.ts, zod.ts, api/add.ts, lib/VideoApiAdapter.ts,
  lib/bbb.ts, lib/getBigBlueButtonAppKeys.ts, static/icon.svg, and
  DESCRIPTION.md.
• Admin-configurable app keys (bbb_url, bbb_secret) following the same
  pattern as Daily / Jitsi.
• lib/bbb.ts implements the standard BBB SHA-1 shared-secret checksum
  per the BBB API security model.
• VideoApiAdapter.createMeeting() calls create then returns a signed
  join URL (redirect=true). deleteMeeting() calls end.
• Registered in all autogenerated app-store registries:
  apps.metadata.generated.ts, apps.server.generated.ts,
  video.adapters.generated.ts, apps.keys-schemas.generated.ts,
  apps.schemas.generated.ts, bookerApps.metadata.generated.ts.
• Added the cal_provide_bigbluebutton_meeting_url locale string.

Closes #1985

Test plan

• Added unit tests in packages/app-store/bigbluebutton/lib/bbb.test.ts
  covering buildQueryString, computeChecksum, and buildApiUrl.
• Maintainer to verify in CI: yarn test packages/app-store/bigbluebutton
  and yarn type-check.
• Maintainer to install the app from /apps, set BBB server URL and
  shared secret in /settings/admin/apps/bigbluebutton, and create a
  booking using the BigBlueButton location.

Notes

• Used the jitsivideo adapter as the reference template (closest analogue: shared-secret auth, dynamic link, no OAuth).
• The video adapter is registered under the key bigbluebutton.
• No new env vars — server URL and shared secret live in the standard
  app.keys admin-settings flow.
• A placeholder icon.svg is included. The maintainer may want to swap it
  for the official BigBlueButton wordmark before merging.

[github-actions]

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