POST /api/sessions/owner-backfill stamps a store's own serving-profile
identity onto its pre-#95407 'profile_name = NULL' session rows. Single
match by construction (each profile's state.db belongs to exactly one
profile), idempotent, one-shot-per-row, never overwrites a non-NULL
owner, and reports the stamped count for logging.
Refs #94724
Teknium