metrics: count LIST membership status, not the subscriber's own status

The first live run recorded newsletter_subs = 0 — a wrong zero, the exact
fail-open this module documents against. Cause: Listmonk's subscriber record
carries status enabled/blocked, while the LIST membership carries
subscription_status confirmed/unconfirmed. Filtering on the subscriber's own
status matched nothing and returned a silent 0.

Now counts memberships whose subscription_status is confirmed (verified: 71,
matching the list), and returns NULL when the endpoint yields no rows at all —
"not measured" rather than "nobody is subscribed". The daily job is the
regression test: it must record 71, not 0.

metrics_snapshot.py

@@ -99,7 +99,7 @@ def from_listmonk() -> int | None:
         print("metrics: LISTMONK_API_TOKEN not set — newsletter_subs left NULL")
         return None
     try:
-        total, page = 0, 1
+        total, page, seen = 0, 1, 0
         while True:
             data = _get_json(
                 f"{LISTMONK_BASE}/api/subscribers?list_id={LISTMONK_LIST_ID}&per_page=100&page={page}",
@@ -107,10 +107,25 @@ def from_listmonk() -> int | None:
             ) or {}
             payload = data.get("data") or {}
             rows = payload.get("results") or []
-            total += sum(1 for s in rows if (s.get("status") or "") == "confirmed")
+            # The subscriber's own `status` is enabled/blocked; the LIST
+            # membership status is what means "on this list" — counting the
+            # former returned a silent, wrong 0.
+            for s in rows:
+                for membership in s.get("lists") or []:
+                    if membership.get("id") == LISTMONK_LIST_ID and (
+                        membership.get("subscription_status") or ""
+                    ) == "confirmed":
+                        total += 1
+            seen += len(rows)
             if not rows or len(rows) < 100 or page >= 20:
                 break
             page += 1
+        if seen == 0:
+            # Nothing came back at all: the endpoint or payload shape changed.
+            # Record "not measured" (NULL) rather than a zero that reads as
+            # "nobody is subscribed".
+            print("metrics: listmonk returned no rows — newsletter_subs left NULL")
+            return None
         return total
     except Exception as exc:  # noqa: BLE001 - report, never guess
         print(f"metrics: listmonk read failed ({exc}) — newsletter_subs left NULL")