Document contacts endpoint in OpenAPI

[caydyan]

Summary

• add the /contacts GET endpoint to docs/deep/openapi.yml
• document supported owner auth headers: x-user-token, x-transport-token, and x-jwt
• document from_group=false and unmet=include query options from contacts.getContacts
• document the success response wrapper with contacts, chats, and subscriptions

Fixes #300

Validation

• ruby -e "require 'yaml'; doc = YAML.load_file('docs/deep/openapi.yml'); abort('missing /contacts') unless doc.fetch('paths').key?('/contacts'); abort('missing get') unless doc['paths']['/contacts'].key?('get'); puts doc['paths']['/contacts']['get']['summary']"
• git diff --check

BTC payout address: bc1qev5ant33v5y89qqjvcf4mh9hlax5svqf5xd7gc

[caydyan]

Current-head validation rerun on 6afa1021c4faa8b8d8144ae81ebf6ff0302cccd6:

• git diff --check origin/master...HEAD passed.
• Structured OpenAPI/YAML validation passed via Ruby YAML.load_file, confirming /contacts exists with the get operation, the expected x-user-token, x-transport-token, and x-jwt auth headers, from_group and unmet query parameters, and a 200 JSON schema containing success plus response.contacts, response.chats, and response.subscriptions.

Local worktree is clean after the rerun.