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

tui/api_client.py

@@ -101,6 +101,16 @@ class MCMApiClient:
     async def get_contacts(self) -> list[dict[str, Any]]:
         return await self._get("/contacts")
 
+    async def get_telegram_qr(self, contact_id: str) -> bytes:
+        """QR-Code PNG-Bytes für einen Kontakt abrufen."""
+        async with httpx.AsyncClient(timeout=10.0, follow_redirects=True) as client:
+            resp = await client.get(
+                self._base + f"/contacts/{contact_id}/telegram-qr",
+                headers=self._headers,
+            )
+            resp.raise_for_status()
+            return resp.content
+
     # ── Status ─────────────────────────────────────────────────────────────────
 
     async def get_channel_status(self) -> dict[str, Any]: