feat(api): add token introspection and self-deletion endpoint (#37995)

Adds a /api/v1/token endpoint that allows tokens to introspect and
delete themselves.
partially fixes: https://github.com/go-gitea/gitea/issues/33583

Assisted-by: Mistral Vibe:mistral-medium-3.5

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/api/v1/swagger/app.go

@@ -20,3 +20,10 @@ type swaggerResponseAccessToken struct {
 	// in:body
 	Body api.AccessToken `json:"body"`
 }
+
+// CurrentAccessToken represents the currently authenticated access token.
+// swagger:response CurrentAccessToken
+type swaggerResponseCurrentAccessToken struct {
+	// in:body
+	Body api.CurrentAccessToken `json:"body"`
+}