GraphQL: add _parent to mutation inputs

[mitselek]

Problem

The create/update mutations don't accept _parent in their input types. There is no way to set parent relationships when creating entities via GraphQL, requiring a REST API fallback.

schema.js:284-293 generates input fields only from user-defined propDefs. System properties are excluded.

Suggested change

schema.js (~line 284):

const inputFields = ['  _parent: [ID]']
for (const p of propDefs) {
  // ... existing logic
}

resolvers.js, create mutation (~line 93):

// Handle _parent from input
if (input._parent) {
  const parents = Array.isArray(input._parent) ? input._parent : [input._parent]
  for (const parentId of parents) {
    properties.push({ type: '_parent', reference: parentId })
  }
}

Same pattern needed in the update mutation.

Example mutation this enables

mutation {
  memberCreate(input: {
    _parent: ["org-id-here"]
    name: "Jane Doe"
    email: "jane@example.com"
  }) {
    _id
    _parent { reference string }
    name { string }
  }
}

Use case

Polyphony PoC: creating a Member under an Organization, or a Role under an Organization, in a single GraphQL mutation without REST fallback.

~15 lines changed, purely additive.

Split from #88

(ER:Codd + Saavedra)