Fix typing and set up pyright - #249

Fixes all the typing errors and enables pyright across the whole package. This required a big refactoring: providers constructed RawCatalogItem classes with all fields nullable that didn't represent nullability correctly and led to many typing errors. The PR drops RawCatalogItem entirely so that providers build CatalogItem directly with proper nullability typing. There are dump() and load() functions to control custom catalog CSV serialization/deserialization.

Also:

• Introduced OfflineProvider and OnlineProvider base classes: primarily to avoid duplicate creds parsing from env for online providers.
• Dropped RunPod spot offers as they'll be removed by the provider soon.