Fix AttributeError when event_date is None in fact_extraction (#875)

* Fix AttributeError when event_date is None in fact_extraction

`_extract_facts_from_chunk` crashes with `'NoneType' object has no
attribute 'isoformat'` when retaining documents without a timestamp.

Two locations fixed:
- Line 1058: debug log called `event_date.isoformat()` without a None
  check
- Line 921: `parse_datetime_flexible()` can return None, so re-check
  before calling `.strftime()` / `.isoformat()`

Fixes #874

* Revert unnecessary None guard on line 921

The original `if event_date is not None:` already guards that block.
Only line 1058 needed the fix.

hindsight-api-slim/hindsight_api/engine/retain/fact_extraction.py

@@ -1055,7 +1055,7 @@ async def _extract_facts_from_chunk(
                     f"LLM response missing 'facts' field or returned empty list. "
                     f"Response: {extraction_response_json}. "
                     f"Input: "
-                    f"date: {event_date.isoformat()}, "
+                    f"date: {event_date.isoformat() if event_date else 'unset'}, "
                     f"context: {context if context else 'none'}, "
                     f"text: {chunk}"
                 )