✨(backend) introduce a configurable analytics system

Add an analytics abstraction that allows configuring which analytics
solution the app uses. PostHog is implemented as one backend, but by
default no analytics backend is activated.

The goal is to track events in a sufficiently organized way and to
let any developer implement their own backend as long as it follows
the same protocol.

src/backend/core/analytics/events.py

@@ -0,0 +1,10 @@
+"""Catalog of all analytics events emitted by the backend."""
+
+from enum import StrEnum
+
+
+class AnalyticsEvent(StrEnum):
+    """All trackable events. Values are the wire names sent to the provider."""
+
+    # Rooms
+    ROOM_CREATED = "room_created"