SQLAlchemy's DateTime(timezone=True) is a no-op on SQLite (the backend
has no native tz type), so values round-tripped through the DB come
back as naive datetimes. The four SQL _row_to_dict helpers were calling
.isoformat() directly on those naive values, shipping timezone-less
strings like "2026-05-20T06:10:22.970977" out of the API. The browser's
new Date(...) then parses them as local time, shifting recent threads
in /threads/search by the local UTC offset (about 8h in Asia/Shanghai).
Route the four call sites through coerce_iso() instead — it already
normalizes naive values as UTC and emits "+00:00" so the wire format
always carries tz. No data migration is needed; existing SQLite rows
read back via the corrected serializer.
PostgreSQL deployments are unaffected because timestamptz preserves
tzinfo end-to-end.
Closes#3120
