fix(ai): count the daily AI budget on the account's own day

The token ledger keyed every day on UTC while the daily briefing and the
other per-day generations run on the account's local day, so east of UTC
the first generations after local midnight were refused against the
previous day's spent ceiling. The budget day is now the calendar day in
the account's time zone; each request still resolves it once and books
its reservation and reconcile on the same row. The medication extraction
reads relative phrases against the same local day, the operator's spend
card shows the operator's own day, and the limit copy says the budget
resets at midnight in the person's time zone.

docs/api/openapi.yaml

@@ -26856,7 +26856,7 @@ components:
             - ko
         today:
           description: "Optional override of the reference date used to resolve relative phrases (\"tomorrow\", \"next Monday\").
-            Format: `YYYY-MM-DD`. Defaults to the server's UTC day."
+            Format: `YYYY-MM-DD`. Defaults to the account's own day in its time zone."
           type: string
           pattern: ^\d{4}-\d{2}-\d{2}$
       required: