metrics: membership history from list-join dates, not welcome_sent_at

The backfill's first real curve was wrong: built on welcome_sent_at it showed
34 members on 19 October, when the co-op had ~65. Most members were provisioned
before the portal sent welcome emails, so that field lags by dozens of people —
and a chart built on it would have misrepresented the co-op's growth.

Membership now comes from Listmonk's per-subscription subscription_created_at
on the members list, which the portal's nightly reconcile keeps mirroring
active members. That reproduces the real curve (13 members mid-September → 71
today). Past days are the cumulative joins of today's members — an honest floor,
documented as such, since members who left are absent.

metrics_snapshot.py

@@ -70,6 +70,42 @@ def _get_json(url: str, headers: dict | None = None):
 
 # ---------------------------------------------------------------- sources
 
+def listmonk_joins() -> list[str] | None:
+    """The date each current member joined the members list (YYYY-MM-DD).
+
+    This is the honest source for a membership curve: the list is maintained by
+    the portal's nightly reconcile to mirror ACTIVE members, and each
+    subscription carries `subscription_created_at`. `welcome_sent_at` in the
+    members table is NOT equivalent — most members were provisioned before the
+    portal sent welcome emails, so a curve built on it lags by dozens of people.
+    """
+    token, user = os.environ.get("LISTMONK_API_TOKEN"), os.environ.get("LISTMONK_API_USER") or "portal-sync"
+    if not token:
+        print("metrics: LISTMONK_API_TOKEN not set — membership history unavailable")
+        return None
+    joins, page = [], 1
+    try:
+        while True:
+            data = _get_json(
+                f"{LISTMONK_BASE}/api/subscribers?list_id={LISTMONK_LIST_ID}&per_page=100&page={page}",
+                {"Authorization": f"token {user}:{token}"},
+            ) or {}
+            rows = ((data.get("data") or {}).get("results")) or []
+            for s in rows:
+                for membership in s.get("lists") or []:
+                    if membership.get("id") == LISTMONK_LIST_ID:
+                        stamp = membership.get("subscription_created_at") or s.get("created_at")
+                        if stamp:
+                            joins.append(str(stamp)[:10])
+            if not rows or len(rows) < 100 or page >= 20:
+                break
+            page += 1
+    except Exception as exc:  # noqa: BLE001
+        print(f"metrics: listmonk joins read failed ({exc})")
+        return None
+    return joins or None
+
+
 def from_sqlite() -> dict:
     """Members, plus the provisioning dates that give the growth curve."""
     conn = connect()
@@ -230,13 +266,14 @@ def snapshot(day: str | None = None) -> dict:
     day = day or today()
     m = from_sqlite()
     subs = from_listmonk()
+    joins = listmonk_joins()
     space = from_matrix()
     spend = spend_by_day(since=dt.date.fromisoformat(day) - dt.timedelta(days=1))
     today_spend = spend.get(day, (None, None))
     row = {
         "members_active": m["active"],
         "members_total": m["total"],
-        "members_new": sum(1 for j in m["joins"] if str(j)[:10] == day),
+        "members_new": (sum(1 for j in joins if j == day) if joins is not None else None),
         "newsletter_subs": subs,
         "matrix_space_members": space,
         "spend_usd": today_spend[0],
@@ -248,29 +285,37 @@ def snapshot(day: str | None = None) -> dict:
 
 
 def backfill() -> None:
-    """Rebuild history: cumulative members from provisioning dates, spend by day."""
+    """Rebuild history: membership from list-join dates, usage from spend logs.
+
+    Membership uses Listmonk's per-subscription `subscription_created_at` — the
+    join date of every CURRENT member. Members who joined and later left are not
+    in it (their departure is likewise absent), so past days are the cumulative
+    joins of today's members: an honest floor for "how many of us were here by
+    that day", not a reconstruction of the roster at the time.
+    """
     m = from_sqlite()
-    joins = sorted(str(j)[:10] for j in m["joins"])
+    joins = sorted(listmonk_joins() or [])
     spend = spend_by_day()
+    if not joins:
+        print("metrics: no membership history (listmonk unavailable) — past days left NULL for members")
     if not joins and not spend:
-        print("metrics: backfill found no history")
+        print("metrics: backfill found no history at all")
         return
 
-    first = min([d for d in joins] + ([min(spend)] if spend else []))
+    candidates = list(joins) + list(spend)
+    first = min(candidates)
     last = today()
     print(
         f"metrics: backfilling {first}..{last} "
-        f"({len(joins)} provisioning dates, {len(spend)} spend days)"
+        f"({len(joins)} join dates, {len(spend)} spend days)"
     )
     day = dt.date.fromisoformat(first)
     end = dt.date.fromisoformat(last)
     written = 0
     while day <= end:
         key = day.isoformat()
-        # Cumulative members AS OF this day, from provisioning dates. Members
-        # provisioned without a welcome email (manual/founder adds) are not in
-        # this history, so backfilled days are a LOWER BOUND — today's row below
-        # carries the exact count.
+        # Cumulative members AS OF this day: current members whose list-join date
+        # is on or before it. Days before anyone joined stay 0.
         members_total = sum(1 for j in joins if j <= key) if joins else None
         row = {
             "members_total": members_total,