fix: log swallowed exceptions in detect_threads try/except (T80.3)

The broad ``except Exception`` around detect_threads silently dropped
programmer errors (wrong kwargs, import-time failures, etc), making
diagnostics painful. Log at DEBUG with full exc_info so the failure
surfaces in local logs without breaking the close pipeline's
failure-tolerant contract.

Adds test_detect_threads_failure_is_logged using caplog.

chat/services/scene_summarize.py

@@ -29,6 +29,7 @@ keeps moving.
 from __future__ import annotations
 
 import json
+import logging
 import uuid
 from datetime import datetime, timezone
 from sqlite3 import Connection
@@ -39,6 +40,8 @@ from chat.eventlog.log import append_and_apply
 from chat.llm.classify import classify
 from chat.llm.client import LLMClient
 
+_log = logging.getLogger(__name__)
+
 
 class ScenePOVSummary(BaseModel):
     """Classifier output: one witness's view of a closing scene.
@@ -589,7 +592,11 @@ async def apply_scene_close_summary(
             open_threads=list_open_threads(conn, chat_id),
             timeout_s=timeout_s,
         )
-    except Exception:
+    except Exception as exc:
+        # T80.3: log the swallowed exception at DEBUG so a
+        # programmer-error flap (e.g. wrong kwarg name) surfaces in
+        # local logs without breaking the close pipeline.
+        _log.debug("detect_threads failed: %s", exc, exc_info=True)
         from chat.services.thread_detection import ThreadDetectionResult
 
         thread_result = ThreadDetectionResult()