Build Coexistence Matrix Endpoint

Build the v2 coexistence-signal class-relatedness matrix in a DETACHED subprocess and write to /data/class_relatedness_matrix_v2.json.

Post-run, the loader picks up the newer disk file (see services.class_relatedness._load_matrix which prefers newer version over the embedded v2-densified fallback).

Wall-clock: ~5-30 minutes depending on marks-table size (~14M rows after backfill). Runs in a detached subprocess like the pipeline endpoints so the HTTP call returns immediately.