Add Dotify audio v2 chunked streaming

[knzeng-e]

Summary

• publish new protected backend uploads as DAV2 chunked encrypted audio refs (dotify:enc:v2:ipfs://...)
• add browser DAV2 parser/decrypter with Range + MSE playback, full-file fallback, and in-stream recovery for recoverable gateway/MSE append failures
• evict failed DAV2 MSE object URLs from the playback cache; chunk authentication failures still fail closed as protected playback errors
• preserve v1 encrypted refs and update artist publish to store full refs while keeping manifest CIDs
• add host startup instrumentation via dotify:host-audio-startup events (source-selected, metadata-ready, first-audio, error)
• document the v2 architecture across the design plan, content protection, README/backlog pages, hook/type references, and docs/reference/audio-v2-container.md
• mark old preview-era backlog notes as superseded by access model v2

Validation

• services/api: npm test
• web: npm run test:unit -- audioV2
• web: npm run build
• PR CI also covers API typecheck/build, web lint/build, EVM compile/test, and e2e checks

[netlify]

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