feat: add api-asset-client crate and use for all model download URLs

[yujonglee]

Summary

• Adds a new api-asset-client crate that fetches model asset metadata (URL, checksum, size) from https://api.hyprnote.com/v1/assets/models
• Uses a cached OnceCell to avoid repeated network calls; exposes a resolve_model convenience function via a global default client
• Updates local-stt plugin to resolve model download URLs from the API, falling back to the hardcoded tar_url/model_url if the API call fails
• Updates local-llm plugin to resolve model download URLs from the API, with the same fallback pattern
• Updates am crate's download method to resolve URLs from the API instead of using hardcoded S3 URLs directly
• Adds strum::Display derive to local-llm::SupportedModel to enable to_string() for asset ID resolution

Review & Testing Checklist for Human

• Verify that the asset IDs produced by to_string() on each model enum (AmModel, WhisperModel, SupportedModel) match the IDs returned by https://api.hyprnote.com/v1/assets/models. Especially SupportedModel variants (Llama3p2_3bQ4, Gemma3_4bQ4, HyprLLM) which use default strum Display without custom serialization
• Test model downloads for each plugin (local-stt, local-llm, am) when the API endpoint is available
• Test model downloads when the API is unreachable to confirm fallback to hardcoded URLs works
• Confirm that am::download being called from local-stt::ext doesn't double-resolve (currently local-stt does its own resolution and doesn't call am::download, so this should be fine)

Notes

Requested by: @yujonglee
Link to Devin run

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