normalize_token_usage_state only length-checked persisted day keys, so a
hand-edited or foreign 10-char key (e.g. "not-a-dat3" or "2026-13-01") in
token-usage.json survived reads and atomic rewrites. token_usage_payload
then parsed every day key with an unguarded datetime.fromisoformat, so one
such key failed every /api/settings and /api/settings/usage request until
the file was repaired by hand.
Validate day keys in normalize_token_usage_state, the shared boundary that
every read, record, and rewrite already funnels through. Malformed keys are
dropped like other malformed rows and scrubbed from the file on the next
write; valid state is unchanged.
