SDKs 0.21.0: preview-enrichment eligibility

All four SDKs now expose previewEnrichmentEligible on VideoDetailDto (GET /videos/{id}, POST /videos/batch) and VideoChangeVideoDto (GET /videos/changes). The field says whether prdb would welcome a preview image for a video from a client that already holds one of its files. It is a server-side selection hint only, not a download instruction and not evidence that a file belongs to the video. Eligibility can disappear after an image is published, a mapping is withdrawn or the video is deleted. Every change of the value reaches the feed, and tombstones carry false.

The generated property is nullable in every language, so a response from an older server that omits it reads as unknown, never as false or as eligible. When the server rule was widened to version 2 (confirmed creator-handle releases now count as well), existing videos changed value without feed events. Consumers adopting the field should take a full /videos/changes baseline. No other schema or operation changed.

A shared contract fixture backs one test per language. Each test covers true/false/absent values on detail and batch responses, upserts and tombstones across SinceId cursor pages, a later flip to false, and the declared 400/403/404 problems.

Python prdb-sdk, TypeScript @prdb/sdk, Go module github.com/prdb-net/prdb-sdk/go (tag go/v0.21.0) and C# Prdb.Sdk share version 0.21.0. Prdb.Hashing is unchanged.

Upstream: prdbnet/prdb!508 and !509 (PRDB-324). Downstream: porganizer#56 and porganizer#59.