fix: skip RESOLUTION section in PSA export for incomplete sessions

Mid-session "Copy for Ticket" no longer shows a misleading Resolution
section that echoed the last step's answer.

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

backend/app/services/export_service.py

@@ -429,7 +429,8 @@ def generate_psa_export(session: Session, options: SessionExport) -> str:
         lines.append("No steps recorded.")
     lines.append("")
 
-    # Resolution
+    # Resolution — only for completed sessions
+    if session.completed_at:
         lines.append("--- RESOLUTION ---")
         _raw_notes = getattr(session, 'outcome_notes', None)
         outcome_notes = _raw_notes if isinstance(_raw_notes, str) else ''