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

.env.example

@@ -0,0 +1,34 @@
+# NtR SoundCloud Fetcher — environment variables
+# Copy to .env and fill in values. Shell env vars override these.
+
+# --- Required -----------------------------------------------------------
+
+NTR_ADMIN_TOKEN=changeme
+
+# --- Server --------------------------------------------------------------
+
+# NTR_HOST=127.0.0.1
+# NTR_PORT=8000
+# NTR_DB_PATH=./ntr_fetcher.db
+# NTR_POLL_INTERVAL_SECONDS=3600
+
+# --- SoundCloud -----------------------------------------------------------
+
+# NTR_SOUNDCLOUD_USER=nicktherat
+
+# --- Show schedule --------------------------------------------------------
+
+# NTR_SHOW_DAY=2                     # 0=Mon … 6=Sun (default: 2=Wed)
+# NTR_SHOW_HOUR=22                   # Hour in Eastern time
+# NTR_SHOW_ROTATION_DELAY_HOURS=0
+
+# --- Dashboard (all three required to enable) -----------------------------
+
+# NTR_WEB_USER=
+# NTR_WEB_PASSWORD=
+# NTR_SECRET_KEY=
+
+# --- Ping (optional defaults for the dashboard ping form) -----------------
+
+# NTR_PING_TARGET=
+# NTR_PING_MESSAGE=