feat: add dashboard ping button and .env file support

Add a "Send IRC Message" section to the admin dashboard that sends a
configurable privmsg to any IRC nick or channel via all connected bots.
New POST /admin/ping endpoint broadcasts a "privmsg" WebSocket message
type, handled by both Limnoria and Sopel plugins.

Also enable pydantic-settings .env file loading (python-dotenv) and
add .env.example documenting all NTR_* configuration variables.

Made-with: Cursor

src/ntr_fetcher/main.py

@@ -85,6 +85,8 @@ def run() -> None:
         web_user=settings.web_user,
         web_password=settings.web_password,
         secret_key=settings.secret_key,
+        ping_target=settings.ping_target,
+        ping_message=settings.ping_message,
     )
 
     @app.on_event("startup")