feat(relay): implement GET /xrpc/com.atproto.server.describeServer (MM-74)
Adds the ATProto service discovery endpoint required by Bluesky clients
during login. Config is extended with available_user_domains (required),
invite_code_required (default true), optional server_did, links, and
contact sections. The handler derives did:web:<host> from public_url as
a placeholder until Wave 3 generates a real DID.
authored by
malpercio.dev
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