[API] add endpoint to check notifications [Extend #9488] (#9595)

* introduce GET /notifications/new

* add TEST

* use Sprintf instead of path.Join

* Error more verbose

* return number of notifications if unreaded exist

* 200 http status for available notifications

templates/swagger/v1_json.tmpl

@@ -494,6 +494,23 @@
         }
       }
     },
+    "/notifications/new": {
+      "get": {
+        "tags": [
+          "notification"
+        ],
+        "summary": "Check if unread notifications exist",
+        "operationId": "notifyNewAvailable",
+        "responses": {
+          "200": {
+            "$ref": "#/responses/NotificationCount"
+          },
+          "204": {
+            "description": "No unread notification"
+          }
+        }
+      }
+    },
     "/notifications/threads/{id}": {
       "get": {
         "consumes": [
@@ -10911,6 +10928,18 @@
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
     },
+    "NotificationCount": {
+      "description": "NotificationCount number of unread notifications",
+      "type": "object",
+      "properties": {
+        "new": {
+          "type": "integer",
+          "format": "int64",
+          "x-go-name": "New"
+        }
+      },
+      "x-go-package": "code.gitea.io/gitea/modules/structs"
+    },
     "NotificationSubject": {
       "description": "NotificationSubject contains the notification subject (Issue/Pull/Commit)",
       "type": "object",
@@ -12397,6 +12426,12 @@
         }
       }
     },
+    "NotificationCount": {
+      "description": "Number of unread notifications",
+      "schema": {
+        "$ref": "#/definitions/NotificationCount"
+      }
+    },
     "NotificationThread": {
       "description": "NotificationThread",
       "schema": {