Add typed class and personal computed-field services

[terjekv]

Gap

Hubuum v0.0.9 exposes 13 computed-field operations, all currently available only through client.openapi.

The missing workflows include:

• class computed-field list/create/get/update/delete;
• expression preview and class-field rebuild;
• current-principal computed-field list/create/get/update/delete and preview.

This is a substantial v0.0.9 data-model capability, but callers currently receive unvalidated dictionaries and must use raw operation IDs.

Acceptance criteria

• Add typed IDs, definition/request/patch, preview, mutation, and computation-state models from the authoritative wire schemas.
• Expose class-scoped computed-field services and a current-principal personal computed-field service.
• Support preview and rebuild without conflating their response types with CRUD.
• Preserve exact sync/async parity.
• Keep expression inputs and computed values out of unsafe representations when they may contain sensitive object data.
• Add unit and pinned-server coverage for class and personal fields, preview, rebuild, and malformed responses.
• Document expression dependencies, rebuild semantics, and personal versus class ownership.