feat(content-node): delegated-routing endpoint used by Helia #64

Summary

Expose Kubo's read-only delegated routing API and configure Helia to find providers through it.

Estimated size: S

Proposal

• Enable the /routing/v1 endpoint on the gateway port (verify the setting name in the pinned release) behind the existing Caddy proxy.
• Configure Helia's routing with @helia/delegated-routing-v1-http-api-client and the endpoint from env.

Acceptance criteria

• Helia finds a provider for a CID pinned on the node without a DHT client.
• The endpoint is read-only.

Dependencies

• feat(infra): run Kubo in docker-compose (server profile, pinned version) #59
• feat(vault): open any CID from a link or paste, with trustless-gateway fallback #54

Part of #43. Ground rules for the content layer: no secrets in the repo, content-layer failures must never block or discard quantum execution, and tests and docs ship with every PR.