Email a DM's recipient when they're genuinely offline (#66)

Scoped to direct messages only, and deliberately narrower than the
existing push/desktop "offline" (not connected to this room's channel
right now, which fires on every message) -- email uses GlobalPresence
instead (no open connection anywhere, or appear_offline), since the
other participant could easily just be active in a different room.
Debounced to the first unread message in the conversation rather than
firing on every message in a burst, resetting once they mark it read.
Reuses the existing SMTP/send_email infrastructure from the invite
feature, so it silently no-ops if SMTP isn't configured, same as
everywhere else that already uses it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/routers/webhooks.py

@@ -28,4 +28,15 @@ async def incoming_webhook_endpoint(
     broadcaster = request.app.state.broadcaster
     presence = request.app.state.presence
     focus_presence = request.app.state.focus_presence
-    await broadcast_new_message(db, broadcaster, presence, focus_presence, room.id, message, sender)
+    global_presence = request.app.state.global_presence
+    await broadcast_new_message(
+        db,
+        broadcaster,
+        presence,
+        focus_presence,
+        global_presence,
+        str(request.base_url),
+        room.id,
+        message,
+        sender,
+    )