feat(sub): expose live online status and add ?format=info endpoint

Custom subscription templates only received the lastOnline timestamp, so
template authors had to fake an online indicator by comparing it against
the current time, and the page was a one-shot server render with no way
to refresh usage without reloading the whole HTML.

The template context (and window.__SUB_PAGE_DATA__) now carries isOnline,
computed from the panel's own online-client tracking (local xray plus
remote nodes) at render time. The subscription URL also answers
?format=info with the page view-model as JSON — minus the links, with
emails deduplicated — so templates can poll live status cheaply. The
shared view-model construction moved into buildSubPageData/subPageContext
so the HTML page, the SPA payload and the info JSON cannot drift apart.

Also documents the previously injected but undocumented announce
template variable.

frontend/public/openapi.json

@@ -10981,7 +10981,7 @@
         "tags": [
           "Subscription Server"
         ],
-        "summary": "Return base64-encoded subscription links for all enabled clients matching the subscription ID. When the request has an Accept: text/html header or ?html=1, renders a styled info page instead. Default path: /sub/:subid.",
+        "summary": "Return base64-encoded subscription links for all enabled clients matching the subscription ID. When the request has an Accept: text/html header or ?html=1, renders a styled info page instead. With ?format=info, returns the page view-model as JSON (traffic, expiry, online status; no links) for live polling. Default path: /sub/:subid.",
         "operationId": "get_subPath_subid",
         "parameters": [
           {
@@ -10993,6 +10993,15 @@
               "type": "string"
             }
           },
+          {
+            "name": "format",
+            "in": "query",
+            "required": false,
+            "description": "Set to \"info\" to get the subscription status view-model as JSON instead of the links.",
+            "schema": {
+              "type": "string"
+            }
+          },
           {
             "name": "subPath",
             "in": "path",