Add scheduled messages and strip markdown from iMessage responses

Strip markdown formatting (bold, italic, headers, code, links, lists) from
LLM responses before sending via iMessage. Add scheduled messages feature
with CRUD API, background scheduler loop, and admin frontend panel.

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

blueprints/imessage/__init__.py

@@ -224,6 +224,8 @@ async def webhook():
         user=user,
     )
 
-    await send_imessage(from_number, response_text)
+    from utils.strip_markdown import strip_markdown
+
+    await send_imessage(from_number, strip_markdown(response_text))
 
     return jsonify({"status": "ok"}), 200