perf(api): add timing diagnostics for media ops

Add monotonic timing instrumentation around media download, semaphore
wait, HTML sanitization, and media file persistence. Log warnings when
operations exceed defined thresholds and provide diagnostic info for
task queue size. This aids performance monitoring without altering
behaviour.

api_server.py

@@ -227,6 +227,8 @@ async def download_media_file(channel: Union[str, int], post_id: int, file_uniqu
     Download media file from Telegram and save to cache
     Returns tuple of (file path, delete_after)
     """
+    import time as _time
+    _fn_start = _time.monotonic()
     base_cache_dir = os.path.abspath("./data/cache")
     
     # Create nested cache structure
@@ -789,8 +791,16 @@ async def get_media(channel: str, post_id: int, file_unique_id: str, digest: str
             channel_id = int(channel)
             
         try: # Wrap the download and prepare call
+            import time as _time
+            _sem_wait_start = _time.monotonic()
             async with HTTP_DOWNLOAD_SEMAPHORE:  # limit concurrent live HTTP downloads
+                _sem_wait = _time.monotonic() - _sem_wait_start
+                if _sem_wait > 0.5:
+                    logger.warning(f"diag_semaphore_wait: {channel}/{post_id}/{file_unique_id} waited {_sem_wait:.3f}s for HTTP_DOWNLOAD_SEMAPHORE")
+                _dl_start = _time.monotonic()
                 file_path, delete_after = await download_media_file(channel_id, post_id, file_unique_id)
+                _dl_elapsed = _time.monotonic() - _dl_start
+                logger.info(f"diag_download_timing: {channel}/{post_id}/{file_unique_id} download_media_file took {_dl_elapsed:.3f}s (semaphore_wait={_sem_wait:.3f}s)")
             if not file_path:
                 raise HTTPException(status_code=404, detail="File not found")
             if file_path: