[Epic] Custom fields — per-workspace issue field definitions + values

Split out of the configurability epic (PAR-189). Per-workspace custom fields on issues.

Scope:

• Schema: custom_field (tenant-scoped: id, name, key, type [text/number/select/date/checkbox], options for selects, position, required) + issue_custom_value (issue_id, field_id, value).
• Backend: CRUD for field definitions (gated on settings.manage), read/write values on issue create/update (respect RBAC: issue.create/update).
• MCP/REST: expose custom values on issue read + accept on create/update.
• UI: a field-definitions admin screen (settings), dynamic field inputs on the issue create/edit forms, and display on the issue detail; filterable later (ties into PAR-191 search).
• Gate visibility behind the per-workspace feature toggle + per-user opt-out from the configurability substrate (PAR-189).

Depends on the settings substrate (PAR-189) for the enable/visibility toggle.