Fix 500 error: replace datetime_range with range in Qdrant filter

qdrant-client 1.15.1 does not support datetime_range in FieldCondition.
Use models.Range with string comparison (same as Lotus reference).
Also wrap date filter in try-except to prevent crash on bad date format.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

search/retrieval.py

@@ -18,15 +18,21 @@ def _build_filter(question: Question) -> models.Filter | None:
     must_conditions: list[models.Condition] = []
 
     if question.date_range:
+        try:
+            must_conditions.append(
+                models.FieldCondition(
+                    key="metadata.end",
+                    range=models.Range(gte=question.date_range.from_),
+                )
+            )
             must_conditions.append(
                 models.FieldCondition(
                     key="metadata.start",
-                datetime_range=models.DatetimeRange(
+                    range=models.Range(lte=question.date_range.to),
-                    gte=question.date_range.from_,
-                    lte=question.date_range.to,
-                ),
                 )
             )
+        except Exception as e:
+            logger.warning("Date filter failed: %s", e)
 
     if question.asker:
         must_conditions.append(