feat(gateway): log upstream requests and responses

[vhuk2025]

Summary

• share a concurrency-safe gateway debug logger between gateway snapshots and HTTP upstream requests
• log redacted upstream request/response metadata and streamed bodies without consuming them

Testing

• GOCACHE=/tmp/sub2api-go-build go test ./internal/repository ./internal/service ./internal/pkg/gatewaydebug

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[vhuk2025]

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