Implements ports.VideoSource against the YouTube Data API v3:
ListSubscriptions (paginated), NewVideos (recent per channel), and
captions-first FetchTranscript — an absent caption track yields
domain.SourceNone (not an error) per ADR-007, with no audio download
or speech-to-text.
OAuth is written fresh on golang.org/x/oauth2 (ADR-006, distinct from
ingestion's inbound MCP auth); the Google token endpoint is inlined to
avoid the heavy x/oauth2/google dep. The per-connection refresh token is
resolved through the SecretStore port from an opaque TokenSecretRef and
is never stored on the adapter or logged.
Unit-tested against an httptest server + fake SecretStore (no live
googleapis egress): subscriptions list/pagination, new-video detection,
captions present -> Source set, captions absent -> SourceNone no error,
and secret-ref resolution failure surfacing as an error.
oauth2 pinned to v0.30.0 to keep the go directive at 1.23.x (koala
runner), not the v0.36 line that requires a newer toolchain.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
