Tailnet-only (Serve, no Funnel) compose variants

Three more composes — compose.tailnet.yml, compose.tailnet.frontend.yml,
compose.tailnet.backend.yml — mirror the funnel set exactly but mount
serve.default/frontend/backend.tailnet.json, which drop AllowFunnel. The
sidecar still registers and serves HTTPS with a tailnet certificate, but
nothing is exposed publicly; Serve also needs no ACL change.

Project names carry a -tailnet suffix so both sets can coexist, and the
README explains that each set needs its own data directory (or host), plus
how to switch a host between funnel and tailnet by starting the other file
with the same .env.

Verified: all six composes validate with docker compose config, and the
six serve configs split cleanly into funnel (AllowFunnel present) and
tailnet-only (absent).

deploy/serve.backend.tailnet.json

@@ -0,0 +1,16 @@
+{
+  "TCP": {
+    "8443": {
+      "HTTPS": true
+    }
+  },
+  "Web": {
+    "${TS_CERT_DOMAIN}:8443": {
+      "Handlers": {
+        "/": {
+          "Proxy": "http://127.0.0.1:80"
+        }
+      }
+    }
+  }
+}