Refactor SDK: entity contracts, new entities, and docs

[martijnbots]

Summary

• Introduce CRUD contracts (Creatable, Deletable, Gettable, Listable, Updatable) as interfaces, replacing the monolithic Entity base class
• Add new entities: Playlist, Playout, and Subtitle with full CRUD support
• Remove MediaClipList (consolidated into Playlist), update Channel and MediaClip with contract-based CRUD
• Rewrite README with comprehensive usage documentation, quick start guide, and examples
• Update license year, improve error handling in Response and HTTPRequestException
• Add unit tests for new entities (PlaylistTest, PlayoutTest, SubtitleTest)

Test plan

• Run composer test to verify all unit tests pass
• Verify new entities (Playlist, Playout, Subtitle) work against SAPI
• Verify existing MediaClip and Channel functionality is not broken
• Check README examples are accurate

🤖 Generated with Claude Code

[ghultink]

Looks good overall — clean contract design, consistent new entities, good security improvement with LIBXML_NONET.

One bug to fix: removing the CRUD methods from the Entity base class introduces an infinite recursion risk. For entities that don't implement CRUD contracts (e.g. Thumbnail), calling $sdk->thumbnail->get(1) will cause AutoAsyncToSyncCaller::__call to check method_exists($this, 'getAsync') → false → falls through to call_user_func_array([$this, 'get'], ...) → triggers __call again → stack overflow.

Previously the base class threw NotImplementedException. Consider adding a guard in __call (e.g. check for recursion or throw a clean error when the async method doesn't exist or re-adding minimal fallbacks to the base class.
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[ghultink]

Additional issue: the Playout entity maps to /sapi/player, but player and playout are separate SAPI entities. This should use /sapi/playout instead.