Reduce logging

[fbac]

Reduce log verbosity for rate limiting, subscriptions, and backfill workers

Downgrades several high-frequency log statements from Info to Debug across multiple packages to reduce noise in production logs.

• In interceptor.go, rate-limited gRPC requests now log at Debug instead of Info.
• In telemetry.go, ResourceExhausted and InvalidArgument errors now log at Debug; all other errors log at Error (previously Info).
• In service.go, subscription start events for group and welcome messages now log at Debug.
• In store.go and the backfiller workers, inbox log insertions, new member entries, and "nothing to process" states now log at Debug.
• Behavioral Change: error-level logging is now used for most gRPC failures in the telemetry interceptor, which is an increase in severity for non-rate-limit errors.

^{Macroscope summarized 31e770e.}

[macroscopeapp]

Approvability

Verdict: Approved

This PR only changes log levels from Info to Debug across multiple files to reduce log verbosity. The telemetry change also upgrades non-rate-limit errors to Error level. All changes are observability-only with no runtime behavior impact, and the author owns all modified files.

^{You can customize Macroscope's approvability policy. Learn more.}

Dismissing prior approval to re-evaluate 31e770e