fix: prune old detail after publication (#100)

src/fenris/collector.py

@@ -477,9 +477,21 @@ def run_collection(
             break
 
         published_count += _recover_pending_for_collection(conn)
+        retention_error = None
+        try:
+            if _pending_count(conn) == 0:
+                from .pruning import prune_old_samples
+
+                prune_old_samples(conn, clock.utcnow())
+        except Exception as exc:  # noqa: BLE001 - publication already committed.
+            # Publication already committed. Keep collection successful and
+            # retry atomic retention on the next normal collection.
+            conn.rollback()
+            retention_error = str(exc)
         return {
             "ok": True,
             "sample_count": published_count,
+            "retention_error": retention_error,
             "store_path": str(store_path),
         }