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

pyproject.toml

@@ -12,6 +12,7 @@ dependencies = [
     "uvicorn[standard]",
     "httpx",
     "pydantic-settings",
+    "python-dotenv",
 ]
 
 [project.optional-dependencies]