The hexagonal interfaces the engine depends on. Keeps the engine provider- and
sink-agnostic: YouTube/Vimeo implement VideoSource, the AI router implements
Summarizer, store/brain implement Sink, ESO implements SecretStore. This is what
makes standalone-vs-homelab a wiring choice (ADR-003).
Pure domain types matching docs/data-model.md: User, Subscription, Video,
Transcript, Summary, plus Provider and TranscriptSource enums. Stdlib-only,
no outward dependencies — the innermost layer. Video carries user_id per the
per-user-isolation decision (no global dedup).
mathias