Replace raw string field names with zf constants in state.go and
db/node.go for consistent, type-safe logging.
state.go changes:
- User creation, hostinfo validation, node registration
- Tag processing during reauth (processReauthTags)
- Auth path and PreAuthKey handling
- Route auto-approval and MapRequest processing
db/node.go changes:
- RegisterNodeForTest logging
- Invalid hostname replacement logging
dd16567c52