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

[lcian]

Replaces the objectstore_service::error::Error enum (which had a variant per failure source — Io, Reqwest, Serde, BackendResponse, GcpAuth, etc.) with an anyhow-shaped Error struct that carries an ErrorKind classification, an optional boxed source error, and an optional context message.

This separates error classification from error origin: backends no longer need to choose between increasingly many enum variants, and the API layer maps a flat ErrorKind to HTTP status codes instead of matching deeply nested enum variants.
The new ErrorKind enum captures the cause of a failure independent of HTTP semantics (ClientStream, InvalidInput, RangeNotSatisfiable, AtCapacity, BackendRateLimited, BackendTimeout, BackendUnavailable, NotImplemented, Internal)

Foreign errors convert via From impls into ErrorKind::Internal with the original as source; the ResultExt trait provides .kind() and .context() to reclassify or annotate without map_err boilerplate.

Closes FS-358

[linear-code]

FS-358