fix(logs): reduce production log spam — agent llm-config polling, scheduler plumbing

- middleware: suppress structured REASON/REFLECT framing for high-frequency
  pollers (/api/agent/llm-config, /api/tasks/{id}, health/summary,
  session/activity, settings/consolidated); fixes tasks/{id} never matching
- agent: _fetch_llm_config treats 401/403 as terminal (no retry, log once),
  bounded backoff 5s/15s/60s on connect/timeout/5xx; langgraph_setup logs
  auth failure once per process
- scheduler: auto-end plumbing lines (executed/scan triggered) -> DEBUG
- thumbnail: Superset 4xx rejections logged at DEBUG instead of EXPLORE

backend/src/app.py

@@ -445,6 +445,30 @@ async def network_error_handler(request: Request, exc: NetworkError):
 
 
 # #endregion App.AppModule.NetworkErrorHandler
+
+# High-frequency polling endpoints whose per-request REASON/REFLECT framing would
+# spam the structured log (they remain visible in the uvicorn access log):
+#   - /api/tasks*            task progress polling (every 1.5s during operations)
+#   - /api/health/summary    health monitoring polling
+#   - /api/agent/llm-config  agent container LLM config polling
+#   - /api/auth/session/activity  session activity heartbeats
+#   - /api/settings/consolidated  settings polling
+_POLLING_EXACT_PATHS = frozenset({
+    "/api/health/summary",
+    "/api/agent/llm-config",
+    "/api/auth/session/activity",
+    "/api/settings/consolidated",
+})
+
+
+def _is_suppressed_request(request: Request) -> bool:
+    """Return True for high-frequency polling requests (framing suppressed)."""
+    path = request.url.path
+    return path in _POLLING_EXACT_PATHS or (
+        request.method == "GET" and path.startswith("/api/tasks")
+    )
+
+
 # #region App.AppModule.LogRequests [C:3] [TYPE Function]
 # @ingroup Module
 # @BRIEF Middleware to log incoming HTTP requests and their response status.
@@ -463,10 +487,7 @@ async def log_requests(request: Request, call_next):
         # Dynamic src derived from the request route — more informative than hardcoded "api.request_handler"
         _route_path = request.url.path.strip("/").replace("/", ".")
         _src = f"route.{request.method}.{_route_path}" if _route_path else f"route.{request.method}.root"
-        is_polling = (
-            (request.url.path.endswith("/api/tasks") and request.method == "GET")
-            or request.url.path.endswith("/api/health/summary")
-        )
+        is_polling = _is_suppressed_request(request)
         if not is_polling:
             logger.reason(
                 "Handle API request",