Add README with copy-paste client install for ntfyschwenkonline.

Documents per-host topic naming and manual token setup via the web UI.

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

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+# ntfy.schwenk.online
+
+Self-hosted [ntfy](https://ntfy.sh) push server behind Traefik.
+
+## Server setup
+
+```bash
+docker compose up -d
+docker exec -it ntfy.schwenk.online ntfy user add frank --role=admin
+```
+
+## Per-host notifications
+
+Each machine posts to a topic derived from its FQDN (`hostname -f`, lowercased, dots → hyphens).
+For example, `web01.example.com` publishes to topic `web01-example-com`.
+
+1. Create a user and access token in the web UI at https://ntfy.schwenk.online
+2. Grant that user read-write access to the host topic (e.g. `web01-example-com`)
+3. Install the command below on the machine, replacing `tk_PASTE_YOUR_TOKEN_HERE`
+
+### Install `ntfyschwenkonline` on a client
+
+Replace the token, then copy and paste the whole block:
+
+```bash
+sudo tee /usr/local/bin/ntfyschwenkonline > /dev/null <<'EOF'
+#!/bin/sh
+set -eu
+
+NTFY_URL="https://ntfy.schwenk.online"
+NTFY_TOKEN="tk_PASTE_YOUR_TOKEN_HERE"
+
+topic() {
+  hostname -f | tr '[:upper:]' '[:lower:]' | sed 's/[^a-z0-9-]/-/g'
+}
+
+if [ "$#" -gt 0 ]; then
+  message="$*"
+elif [ -t 0 ]; then
+  echo "usage: ntfyschwenkonline [message]" >&2
+  echo "       echo message | ntfyschwenkonline" >&2
+  exit 1
+else
+  message=$(cat)
+fi
+
+curl -fsS \
+  -H "Authorization: Bearer ${NTFY_TOKEN}" \
+  -H "Title: $(hostname -s)" \
+  -d "${message}" \
+  "${NTFY_URL}/$(topic)"
+EOF
+sudo chmod 755 /usr/local/bin/ntfyschwenkonline
+```
+
+### Use
+
+```bash
+ntfyschwenkonline "backup finished"
+echo "disk 95% full" | ntfyschwenkonline
+```
+
+Messages appear on the host topic in the ntfy web UI or any subscriber.