feat(HNT-2887): add crawl domain message and event types

[mmiermans]

Goal

HNT-2887

Add the discovery-side and agent-side crawl domain types so the validators, handlers, and consumers that follow reason against one shared set of interfaces. This slice introduces DiscoveryContext, CrawlArticleDiscoveryMessage, LiveArticle, and PublisherList in the messages module, ArticleDiscoveryEvent in the events module, and an optional refresh_interval_minutes on CrawlArticleMessage, all re-exported from the crawl-common barrel. The shapes to reason about are the discovery job, the discovery event that becomes a crawl.article_discoveries row, and the publisher list the agent loads from JSON.

See docs/ARCHITECTURE.md in #14 for the message and event contracts these types encode.

Implementation decisions

Decision	Approach	Why
Make refresh_interval_minutes optional on CrawlArticleMessage	Declare it as an optional field, and the worker falls back to its configured fetch TTL when it is absent	A rolling deploy must not reject messages that were enqueued before the field existed, which is the additive-only schema evolution that Google Cloud Pub/Sub schema evolution is designed to support: add the field, keep it optional, then depend on it.