feat(api): list all packages for site administrators (#38968)

Add `GET /admin/packages` so site administrators can review packages
across every owner without querying each owner separately.

It returns the same package version representation as `GET
/packages/{owner}` and supports `page`, `limit`, `type`, and `q`
filters.

---
Assisted by Codet(DeepSeek)

---------

Signed-off-by: bircni <bircni@icloud.com>
Signed-off-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/swagger/v1-openapi3.generated.json

@@ -11685,6 +11685,81 @@
         ]
       }
     },
+    "/admin/packages": {
+      "get": {
+        "operationId": "adminListPackages",
+        "parameters": [
+          {
+            "description": "page number of results to return (1-based)",
+            "in": "query",
+            "name": "page",
+            "schema": {
+              "type": "integer"
+            }
+          },
+          {
+            "description": "page size of results",
+            "in": "query",
+            "name": "limit",
+            "schema": {
+              "type": "integer"
+            }
+          },
+          {
+            "description": "package type filter",
+            "in": "query",
+            "name": "type",
+            "schema": {
+              "enum": [
+                "alpine",
+                "cargo",
+                "chef",
+                "composer",
+                "conan",
+                "conda",
+                "container",
+                "cran",
+                "debian",
+                "generic",
+                "go",
+                "helm",
+                "maven",
+                "npm",
+                "nuget",
+                "pub",
+                "pypi",
+                "rpm",
+                "rubygems",
+                "swift",
+                "terraform",
+                "vagrant"
+              ],
+              "type": "string"
+            }
+          },
+          {
+            "description": "name filter",
+            "in": "query",
+            "name": "q",
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "$ref": "#/components/responses/PackageList"
+          },
+          "403": {
+            "$ref": "#/components/responses/forbidden"
+          }
+        },
+        "summary": "List all packages",
+        "tags": [
+          "admin"
+        ]
+      }
+    },
     "/admin/unadopted": {
       "get": {
         "operationId": "adminUnadoptedList",