ref(service): Replace Error with anyhow-like struct with embedded kind

[lcian]

Replaces the objectstore_service::error::Error enum with a struct containing an independent ErrorKind classification and an anyhow::Error diagnostic.

This separates error classification from error origin: anyhow preserves the source and stacked context chain, while backends can reclassify errors with .kind() and add context with .context() without map_err boilerplate. The API maps the flat ErrorKind to HTTP status codes.

Closes FS-358
Ref FS-439

[cursor]

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