fix(aider-watch): code-review fixes (4 issues)

1. aiderw: session_end 補 model+cwd (AI Router feedback loop 修通)
2. repository: model_stats_since SQL 改 COALESCE(session_end, session_start) model
3. aider_event_service: classify_severity 移除 error_count 觸發告警（防假陽性）
4. worker: run_aider_event_processor_loop 包 proc.start() try/except（防靜默崩潰）

2026-04-20 @ Asia/Taipei

apps/api/src/workers/aider_event_processor.py

@@ -190,7 +190,11 @@ def get_aider_event_processor() -> AiderEventProcessor:
 async def run_aider_event_processor_loop() -> None:
     """main.py lifespan 呼叫此函式。"""
     proc = get_aider_event_processor()
-    await proc.start()
+    try:
+        await proc.start()
+    except Exception:
+        logger.exception("aider_processor_start_failed")
+        return
     try:
         while proc._running:
             await asyncio.sleep(60)