Initial transfer API rail: client, router, docs fetcher, tests

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.example

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+# Transfer API Rail — external ISO 20022 API
+# Copy to .env and set values. Do not commit .env.
+
+# Base URL of the external API (e.g. IPv4 address)
+TRANSFER_RAIL_BASE_URL=http://187.43.157.150
+
+# API key (e.g. IPv6 address or token provided by the provider)
+TRANSFER_RAIL_API_KEY=
+
+# Optional: path to API docs on the external host (default: /openapi.json)
+TRANSFER_RAIL_DOCS_PATH=/openapi.json
+
+# Optional: header name for API key (default: X-API-Key)
+TRANSFER_RAIL_API_KEY_HEADER=X-API-Key