perf(downloads): implement concurrent download queue with semaphore limiting and retry logic

Add queue-based background download system to improve performance and reliability:
- Introduce DOWNLOAD_SEMAPHORE to limit concurrent downloads to 3
- Add asyncio.Queue (maxsize 100) with dedicated worker for background processing
- Implement safe_get_messages and safe_download_media wrappers with timeout protection (30s and 120s)
- Add retry logic for KeyError auth failures with 5s backoff
- Replace synchronous sequential downloads with asynchronous queued processing
- Prevent event loop blocking by queuing files instead of immediate download

telegram_client.py

@@ -83,6 +83,36 @@ class TelegramClient:
             # Emergency termination
             os._exit(1)
 
+    async def safe_get_messages(self, channel_id, post_id, max_retries=2):
+        """Wrapper with retry logic for auth errors"""
+        for attempt in range(max_retries):
+            try:
+                return await asyncio.wait_for(
+                    self.client.get_messages(channel_id, post_id),
+                    timeout=30.0
+                )
+            except Exception as e:
+                if "KeyError" in str(e) and attempt < max_retries - 1:
+                    logger.warning(f"Auth error on attempt {attempt + 1}, retrying in 5s...")
+                    await asyncio.sleep(5)
+                    continue
+                raise
+
+    async def safe_download_media(self, file_id, file_name, max_retries=2):
+        """Wrapper with retry logic for download errors"""
+        for attempt in range(max_retries):
+            try:
+                return await asyncio.wait_for(
+                    self.client.download_media(file_id, file_name=file_name),
+                    timeout=120.0
+                )
+            except Exception as e:
+                if "KeyError" in str(e) and attempt < max_retries - 1:
+                    logger.warning(f"Download auth error on attempt {attempt + 1}, retrying...")
+                    await asyncio.sleep(5)
+                    continue
+                raise
+
     async def stop(self):
         if self.client.is_connected:
             await self.client.stop()