Refactor code structure for improved readability and maintainability

docs/api/gateway/export-center.md

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+# Export Center Gateway Contract (draft placeholder)
+
+**Status:** TODO · awaiting Export Center Guild inputs
+
+## Scope
+- Profile, run, download, and distribution routes proxied via Web gateway.
+- Tenant scoping, RBAC/ABAC, streaming limits, retention/encryption parameters, signed URL policy.
+
+## Needed from owners
+- OpenAPI/JSON schema for: profiles, runs, downloads, distributions (OCI/object storage).
+- Range/streaming limits; retry/backoff guidance; checksum/manifest format.
+- Required headers (tenant/project, idempotency, auth) and rate limits.
+- Example payloads/NDJSON streams for happy-path and error cases.
+
+## TODO
+- Replace this file with the ratified contract and sample payloads.
+- Record schema hash and date when published; link from Web II sprint Execution Log.