HandleAttestations routes GET (query) and POST (create) for /api/attestations.
GET returns attestations matching optional filters (JSON array).
Query parameters:
?subject=x — filter by subject(s), comma-separated
?predicate=y — filter by predicate(s), comma-separated
?context=z — filter by context(s), comma-separated
?actor=a — filter by actor(s), comma-separated
?source=s — filter by source (exact match, e.g. "levi", "cli")