feat: add systemd services and nginx config

- signal-cli-bot.service: runs signal-cli daemon on 127.0.0.1:8081
- github-to-signal.service: depends on signal-cli-bot, auto-starts it
- nginx reverse proxy with TLS termination
- README updated with full deployment instructions

deploy/github-to-signal.nginx.conf

@@ -0,0 +1,28 @@
+server {
+    listen 443 ssl;
+    server_name ghwebhook.example.com;
+
+    ssl_certificate     /etc/letsencrypt/live/ghwebhook.example.com/fullchain.pem;
+    ssl_certificate_key /etc/letsencrypt/live/ghwebhook.example.com/privkey.pem;
+
+    location /webhook {
+        proxy_pass http://127.0.0.1:9900;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+
+        # GitHub webhooks are small and fast
+        proxy_read_timeout 30s;
+        client_max_body_size 1m;
+    }
+
+    location /health {
+        proxy_pass http://127.0.0.1:9900;
+    }
+
+    # Block everything else
+    location / {
+        return 404;
+    }
+}