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/feedback/sql.py

@@ -13,6 +13,7 @@ from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
 
 from deerflow.persistence.feedback.model import FeedbackRow
 from deerflow.runtime.user_context import AUTO, _AutoSentinel, resolve_user_id
+from deerflow.utils.time import coerce_iso
 
 
 class FeedbackRepository:
@@ -24,7 +25,8 @@ class FeedbackRepository:
         d = row.to_dict()
         val = d.get("created_at")
         if isinstance(val, datetime):
-            d["created_at"] = val.isoformat()
+            # SQLite drops tzinfo on read; normalize via ``coerce_iso`` so output is always tz-aware.
+            d["created_at"] = coerce_iso(val)
         return d
 
     async def create(