Introduce a backward-compatible item kind discriminator (experience, talk, presentation, movie) and first-class media assets. Talks require one or more ordered assets; presentation/movie items may be standalone. Asset records include role, format, original name, durable URI/path, size, SHA-256, source transfer ID, and optional Google Slides source URL/export metadata. Add migrations and round-trip tests for existing YAML data.
Acceptance: existing experience clients keep working; API/CLI/UI can CRUD all four kinds; talk cardinality and supported formats are validated.