Add transcript-backed YouTube shadow ingestion

## Parent PRD

#1

## What to build

Add a transcript-backed YouTube ingestion path in shadow mode using a server-side API-key-based provider. This slice should fetch transcript-enriched YouTube source material, run it through the new matching/publication gates, and surface candidates and failures internally without yet publishing new public stalls. This is a human-in-the-loop slice because provider selection and API key setup require external coordination.

## Acceptance criteria

- [ ] A transcript-capable YouTube source adapter works with a server-side API key provider compatible with the deployment model.
- [ ] Transcript-backed YouTube candidates can be produced and evaluated internally without public auto-publication.
- [ ] Tests cover transcript-gated candidate generation and observable failure handling when transcripts are unavailable or unusable.

## Blocked by

- Blocked by #7

## User stories addressed

Reference by number from the parent PRD:

- User story 16
- User story 19
- User story 20
- User story 22
- User story 30