Set Processor Eval Rubric

Link or detach an EvalRubric on a custom processor.

DUAL WRITE (mirrors the generation-agent flow in custom_processors/tasks.py, where the version's eval_definition and the processor.eval_rubric_id back-link are always written together): the execution engine reads ONLY the rubric copy embedded in the ACTIVE version's eval_definition, so the back-link and that copy must move as one.

• {"eval_rubric_id": <id>}: verifies the rubric exists on this team (404) and isn't archived (400), then sets processor.eval_rubric_id AND mirrors rubric.eval_definition into the active version. Requires a completed active version (400 otherwise) — a link without the mirrored copy would silently never score.
• {"eval_rubric_id": null}: clears processor.eval_rubric_id AND the active version's eval_definition. Tolerates a processor with no active version (nothing to clear on the version side), so a dangling link is always removable.