Migration 0010 widens the existing operators table for username +
password auth. Backfills 'admin' on the seed row so the NOT NULL
constraint succeeds; password_hash stays nullable so the operator is
forced to set one via scripts/set-password.sh before they can sign in.
Adds a unique index on lower(username).
seed.ts also picks up the new username field (defaults to 'admin' so
re-running scripts/db.sh seed stays idempotent against the backfilled row).
