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

api/routes/contacts.py

@@ -1,7 +1,12 @@
+import io
+
+import qrcode
 from fastapi import APIRouter, Depends, HTTPException, status
+from fastapi.responses import StreamingResponse
 from sqlalchemy.orm import Session
 
 from api.auth import require_api_key
+from config import settings
 from db.database import get_db
 from schemas import ContactCreate, ContactResponse, ContactUpdate
 from services import contact_service
@@ -58,3 +63,28 @@ def delete_contact(
     if not contact:
         raise HTTPException(status_code=404, detail="Contact not found")
     contact_service.delete(db, contact)
+
+
+@router.get("/{contact_id}/telegram-qr", tags=["contacts"])
+def telegram_qr(
+    contact_id: str,
+    db: Session = Depends(get_db),
+    _: str = Depends(require_api_key),
+):
+    """QR-Code als PNG: Invite-Link für Telegram-Bot mit Kontakt-ID als Deep-Link-Parameter."""
+    contact = contact_service.get_by_id(db, contact_id)
+    if not contact:
+        raise HTTPException(status_code=404, detail="Contact not found")
+
+    bot_username = settings.telegram_bot_username.lstrip("@")
+    invite_url = f"https://t.me/{bot_username}?start={contact_id}"
+
+    img = qrcode.make(invite_url)
+    buf = io.BytesIO()
+    img.save(buf, format="PNG")
+    buf.seek(0)
+    return StreamingResponse(
+        buf,
+        media_type="image/png",
+        headers={"Content-Disposition": f'attachment; filename="telegram_qr_{contact_id}.png"'},
+    )