Initial MCM project: FastAPI + Textual TUI unified messenger

MultiCustomerMessenger supporting Telegram (python-telegram-bot),
WhatsApp (Green API) and SMS (python-gsmmodem-new). REST API with
Bearer-token auth, SQLAlchemy models for MariaDB, APScheduler for
background polling, and Textual TUI running in same asyncio event-loop.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

channels/base.py

@@ -0,0 +1,37 @@
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from typing import Any
+
+
+class BaseChannel(ABC):
+    """Abstrakte Basisklasse für alle Messaging-Kanäle."""
+
+    @abstractmethod
+    async def send_message(
+        self,
+        recipient: str,
+        text: str,
+        reply_to_id: str | None = None,
+    ) -> dict[str, Any]:
+        """Nachricht senden.
+
+        Returns:
+            {"success": bool, "channel_message_id": str | None, "error": str | None}
+        """
+
+    @abstractmethod
+    async def check_connection(self) -> tuple[bool, str]:
+        """Verbindung prüfen.
+
+        Returns:
+            (connected: bool, detail: str)
+        """
+
+    @abstractmethod
+    async def start(self) -> None:
+        """Kanal starten (Webhook registrieren, Polling starten, …)."""
+
+    @abstractmethod
+    async def stop(self) -> None:
+        """Kanal sauber beenden."""