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

services/message_service.py

@@ -109,11 +109,29 @@ async def handle_inbound(payload: dict[str, Any]) -> None:
         channel = payload["channel"]
 
         if channel == "telegram":
-            contact = contact_service.get_or_create_by_telegram(
-                db,
-                payload["sender_telegram_id"],
-                name=payload.get("sender_name", "Unknown"),
-            )
+            link_contact_id = payload.get("link_contact_id")
+            if link_contact_id:
+                # Kontakt aus QR-Code: telegram_id verknüpfen
+                existing = contact_service.get_by_id(db, link_contact_id)
+                if existing:
+                    from schemas import ContactUpdate
+                    contact_service.update(
+                        db, existing,
+                        ContactUpdate(telegram_id=payload["sender_telegram_id"])
+                    )
+                    contact = existing
+                else:
+                    contact = contact_service.get_or_create_by_telegram(
+                        db,
+                        payload["sender_telegram_id"],
+                        name=payload.get("sender_name", "Unknown"),
+                    )
+            else:
+                contact = contact_service.get_or_create_by_telegram(
+                    db,
+                    payload["sender_telegram_id"],
+                    name=payload.get("sender_name", "Unknown"),
+                )
             channel_conv_id = payload["chat_id"]
         else:
             contact = contact_service.get_or_create_by_phone(