feat: local batch transcription with SSE progress for Cactus

[yujonglee]

Summary

• Adds streaming batch transcription support for the Cactus adapter via SSE (Server-Sent Events), enabling real-time progress reporting during local batch transcription
• Extracts StreamingBatchEvent and StreamingBatchStream types from argmax-specific code into shared adapter module so both Argmax and Cactus adapters can use them
• Refactors transcribe-cactus service layer: extracts TranscribeService + TranscribeServiceBuilder into streaming/service.rs and adds build_metadata to a shared service/mod.rs; the batch HTTP handler now supports both plain response and SSE streaming based on the Accept: text/event-stream header

Test plan

• Verify local batch transcription via Cactus adapter emits progress percentage events over SSE
• Verify WebSocket streaming path still works after TranscribeService refactor
• Run cargo check to confirm no compilation errors

Made with Cursor

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