feat(api): add client lookup by subscription ID (#6908)

Integrations could only find a client by subscription ID through a substring
search over the paged client list followed by a second lookup by email.
GET /panel/api/clients/get/subId/:subId now returns the full client records
(settings, attached inbounds, external links, traffic, tunnel addresses) for an
exact subId match, as a list because older rows may share an ID. It sits behind
the same admin login or admin-token check as the email lookup.

docs/public/openapi.json

@@ -8501,6 +8501,81 @@
         }
       }
     },
+    "/panel/api/clients/get/subId/{subId}": {
+      "get": {
+        "tags": [
+          "Clients"
+        ],
+        "summary": "Fetch clients by subscription ID.",
+        "operationId": "get_panel_api_clients_get_subId_subId",
+        "description": "Matches the exact subscription ID and orders results by client ID. Returns an array of the same payloads as GET /clients/get/:email, since multiple clients can share a subscription ID. Includes disabled, expired and unattached clients. An unknown ID returns HTTP 200 with success:false, obj:null and a record-not-found message, matching the email lookup. Requires an admin token or a panel login session.",
+        "parameters": [
+          {
+            "name": "subId",
+            "in": "path",
+            "required": true,
+            "description": "Exact subscription ID (URL-encoded).",
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Successful response",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "object",
+                  "properties": {
+                    "success": {
+                      "type": "boolean"
+                    },
+                    "msg": {
+                      "type": "string"
+                    },
+                    "obj": {
+                      "type": "array",
+                      "nullable": true,
+                      "items": {
+                        "type": "object",
+                        "properties": {
+                          "client": {
+                            "$ref": "#/components/schemas/ClientRecord"
+                          },
+                          "inboundIds": {
+                            "type": "array",
+                            "items": {
+                              "type": "integer"
+                            }
+                          },
+                          "externalLinks": {
+                            "type": "array",
+                            "items": {
+                              "type": "object"
+                            }
+                          },
+                          "usedTraffic": {
+                            "type": "integer",
+                            "format": "int64"
+                          },
+                          "tunnelAllowedIPs": {
+                            "type": "object",
+                            "additionalProperties": {
+                              "type": "string"
+                            }
+                          }
+                        }
+                      }
+                    }
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/panel/api/clients/get/tgId/{tgId}": {
       "get": {
         "tags": [