fix(persistence): emit tz-aware timestamps from SQLite-backed stores (#3130)

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

backend/packages/harness/deerflow/persistence/thread_meta/sql.py

@@ -13,6 +13,7 @@ from deerflow.persistence.json_compat import json_match
 from deerflow.persistence.thread_meta.base import InvalidMetadataFilterError, ThreadMetaStore
 from deerflow.persistence.thread_meta.model import ThreadMetaRow
 from deerflow.runtime.user_context import AUTO, _AutoSentinel, resolve_user_id
+from deerflow.utils.time import coerce_iso
 
 logger = logging.getLogger(__name__)
 
@@ -28,7 +29,9 @@ class ThreadMetaRepository(ThreadMetaStore):
         for key in ("created_at", "updated_at"):
             val = d.get(key)
             if isinstance(val, datetime):
-                d[key] = val.isoformat()
+                # SQLite drops tzinfo despite ``DateTime(timezone=True)``;
+                # ``coerce_iso`` normalizes naive values as UTC so the wire format always carries tz.
+                d[key] = coerce_iso(val)
         return d
 
     async def create(