chart: inject TOWER_VERSION env from image tag

Lets the substrate endpoint surface the running tag without depending on Go ldflag tricks. Single source of truth: bumping the chart tag automatically bumps what the UI displays.

templates/backend-deployment.yaml

@@ -33,6 +33,8 @@ spec:
           env:
             - name: TOWER_LISTEN_ADDR
               value: ":8080"
+            - name: TOWER_VERSION
+              value: {{ .Values.backend.image.tag | quote }}
             - name: TOWER_GITEA_URL
               value: {{ .Values.config.giteaURL | quote }}
             - name: TOWER_GITEA_ORG