Fix ConnectError handling in Telegram bot API client

Fix AttributeError crash when backend is unreachable during account linking.
Previously, when telegram bot couldn't connect to backend, the error handler
tried to access e.response.status_code on a ConnectError exception which
doesn't have a response attribute.

Changes to reports-app/telegram-bot/app/api/client.py:
- Import ConnectError from httpx
- Add separate exception handler for ConnectError before HTTPError handler
- Log clear error message indicating backend connectivity issue
- Return None gracefully instead of crashing with AttributeError

Changes to deployment/windows/docs/TELEGRAM_BOT_TROUBLESHOOTING.md:
- Add new section "Problem: Cannot connect to backend / Connection Errors"
- Add diagnostic steps for backend service verification
- Add checklist for BACKEND_URL configuration (http://localhost:8000)
- Add Issue 5: Backend Service Not Running
- Add Issue 6: Wrong Backend URL in Telegram Bot
- Include PowerShell commands for Windows Server troubleshooting

This fix ensures the Telegram bot provides clear error messages when backend
is unavailable instead of crashing, making debugging easier for production
deployments.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

reports-app/telegram-bot/app/api/client.py

@@ -11,7 +11,7 @@ from typing import Optional, Dict, Any, List
 from datetime import datetime
 
 import httpx
-from httpx import AsyncClient, Response, HTTPError
+from httpx import AsyncClient, Response, HTTPError, ConnectError
 
 logger = logging.getLogger(__name__)
 
@@ -144,6 +144,10 @@ class BackendAPIClient:
 
             return await self._handle_response(response)
 
+        except ConnectError as e:
+            logger.error(f"Cannot connect to backend at {self.base_url}: {e}")
+            logger.error("Verify that backend service is running and BACKEND_URL is correct")
+            return None
         except HTTPError as e:
             if e.response.status_code == 404:
                 logger.warning(f"User {oracle_username} not found in Oracle")