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/run/sql.py

@@ -17,6 +17,7 @@ from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
 from deerflow.persistence.run.model import RunRow
 from deerflow.runtime.runs.store.base import RunStore
 from deerflow.runtime.user_context import AUTO, _AutoSentinel, resolve_user_id
+from deerflow.utils.time import coerce_iso
 
 
 class RunRepository(RunStore):
@@ -68,11 +69,13 @@ class RunRepository(RunStore):
         # Remap JSON columns to match RunStore interface
         d["metadata"] = d.pop("metadata_json", {})
         d["kwargs"] = d.pop("kwargs_json", {})
-        # Convert datetime to ISO string for consistency with MemoryRunStore
+        # Convert datetime to ISO string for consistency with MemoryRunStore.
+        # SQLite drops tzinfo on read despite ``DateTime(timezone=True)`` —
+        # ``coerce_iso`` normalizes naive datetimes as UTC.
         for key in ("created_at", "updated_at"):
             val = d.get(key)
             if isinstance(val, datetime):
-                d[key] = val.isoformat()
+                d[key] = coerce_iso(val)
         return d
 
     async def put(