feat: Telegram QR-Code Invite-Link + WhatsApp Empfang-Fix

Telegram:
- /start <contact_id> Deep-Link Handler: verknüpft Kontakt automatisch mit chat_id
- QR-Code Endpunkt GET /api/v1/contacts/{id}/telegram-qr (PNG)
- TUI: Taste T öffnet QR-Code im Browser (HTML mit eingebettetem PNG)
- config.py + .env.example: TELEGRAM_BOT_USERNAME=mcm_bot
- qrcode[pil] zu requirements.txt hinzugefügt

WhatsApp:
- receiveTimeout 5→3s, HTTP-Timeout 8s → verhindert Polling-Overlap

channels/whatsapp_channel.py

@@ -85,7 +85,8 @@ class WhatsAppChannel(BaseChannel):
             return
         url = f"{self._base_url}/receiveNotification/{self._token}"
         try:
-            async with self._get_session().get(url, params={"receiveTimeout": 5}) as resp:
+            timeout = aiohttp.ClientTimeout(total=8)
+            async with self._get_session().get(url, params={"receiveTimeout": 3}, timeout=timeout) as resp:
                 if resp.status != 200:
                     return
                 data = await resp.json()