feat: install nodes and the backend from a static compose file; remove dead code [CI]

Agent, Probe and Backend now start from one static compose.yaml plus a local
.env: the panel (or the release assets, for the very first Backend) provides
the file, the panel's command writes the public values into .env, checks the
file's SHA-256 and runs compose up. Updating is compose pull && compose up -d.
The image is chosen by the CI channel (:latest, or MOSHIRO_VERSION in .env),
not by the panel; the previous one-line script stays as a fallback.

- packaging/compose/{agent,probe,backend}/compose.yaml are the single source,
  embedded in the Backend and served at /install/compose/{file}; the root
  compose.yaml moved there. Releases also carry them and list them in the
  signed SHA256SUMS.
- The container entrypoint defaults the node name to the host name.
- Join token creation returns a compose block (.env, url, hash, per-platform
  commands); the admin dialog leads with it.
- Remove code with no references (HotLayer members, LiveHub.SubscriberCount,
  QueryService.ParseParams, ReleaseService.Forget, a few constants) and reduce
  the install-code lookup to the role it is for.

backend/openapi/v1.json

@@ -4966,6 +4966,40 @@
         }
       }
     },
+    "/install/compose/{file}": {
+      "get": {
+        "tags": [
+          "ops"
+        ],
+        "summary": "The static compose file of agent, probe or backend (agent.yaml, probe.yaml, backend.yaml; same bytes for everyone)",
+        "operationId": "install_compose",
+        "parameters": [
+          {
+            "name": "file",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "OK"
+          },
+          "404": {
+            "description": "Not Found",
+            "content": {
+              "application/problem+json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDto"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/install.ps1": {
       "get": {
         "tags": [
@@ -5796,6 +5830,37 @@
           }
         }
       },
+      "ComposeInstallDto": {
+        "required": [
+          "env",
+          "url",
+          "sha256",
+          "linux",
+          "macos",
+          "windows"
+        ],
+        "type": "object",
+        "properties": {
+          "env": {
+            "type": "string"
+          },
+          "url": {
+            "type": "string"
+          },
+          "sha256": {
+            "type": "string"
+          },
+          "linux": {
+            "type": "string"
+          },
+          "macos": {
+            "type": "string"
+          },
+          "windows": {
+            "type": "string"
+          }
+        }
+      },
       "ConfirmBackendUpdateRequest": {
         "required": [
           "seq"
@@ -6029,7 +6094,8 @@
         "required": [
           "token",
           "plaintext",
-          "install_commands"
+          "install_commands",
+          "compose"
         ],
         "type": "object",
         "properties": {
@@ -6041,6 +6107,9 @@
           },
           "install_commands": {
             "$ref": "#/components/schemas/InstallCommandsDto"
+          },
+          "compose": {
+            "$ref": "#/components/schemas/ComposeInstallDto"
           }
         }
       },