debug: add stderr logging to _poll_task for every status poll attempt

Logs [poll] lines to stderr so Claude Desktop's MCP log shows exactly
what DSM returns on each status call: attempt number, elapsed time,
finished flag, data keys (on success) or error code + message (on 599).

Version 0.2.3 — remove this logging once the 599 root cause is confirmed.

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

src/mcp_synology_filestation/tools/filestation.py

@@ -81,17 +81,26 @@ def register_filestation(
             ``(True, status_dict)`` on success, or ``(False, "Error: …")`` on
             DSM error or timeout.
         """
+        import sys as _sys
+
         from mcp_synology_filestation.client import SynologyError as _SynologyError
 
         delay = 0.2
         elapsed = initial_delay
         timeout = 60.0
         consecutive_599 = 0
+        attempt = 0
+
+        _sys.stderr.write(
+            f"[poll] START {api}/status v{version} taskid={taskid} initial_delay={initial_delay}\n"
+        )
+        _sys.stderr.flush()
 
         if initial_delay > 0:
             await asyncio.sleep(initial_delay)
 
         while True:
+            attempt += 1
             try:
                 status_data = await client.request(
                     api,
@@ -100,7 +109,18 @@ def register_filestation(
                     params={"taskid": taskid},
                 )
                 consecutive_599 = 0
+                finished = status_data.get("finished")
+                _sys.stderr.write(
+                    f"[poll] #{attempt} elapsed={elapsed:.2f}s finished={finished}"
+                    f" data_keys={list(status_data.keys())}\n"
+                )
+                _sys.stderr.flush()
             except _SynologyError as e:
+                _sys.stderr.write(
+                    f"[poll] #{attempt} elapsed={elapsed:.2f}s"
+                    f" SynologyError code={e.code} msg={e}\n"
+                )
+                _sys.stderr.flush()
                 if e.code == 599:
                     # 599 can be transient (task just started, not yet available).
                     # Retry up to 5 times before giving up.