metrics: daily snapshot of members and usage, backfillable, plus a read route

The co-op had no longitudinal data: every number was an ad-hoc query, so
nothing compounded and no single source owned "how many members do we have,
and when did that change". That cost us a wrong figure in the newsletter ("six
new members" — actually 21; the six were forum joins).

New: metrics_snapshot.py writes one row per day into the portal's SQLite store
(the same members.db), and GET /admin/metrics/series returns them oldest-first
for the panel and dashboard to chart later.

  - Cumulative columns (members_active, members_total, members_new,
    newsletter_subs, matrix_space_members) are counts AS OF that day;
    spend_usd/requests are THAT day's traffic.
  - FAIL CLOSED PER METRIC: an unreadable source yields NULL, never a zero — a
    zero reads as "nobody" and would bake a lie into a chart. Failures print to
    stdout, and a scheduler task's stdout lands in the app log.
  - Listmonk's cached subscriber_count is NOT trusted (it reads 142 for a list
    holding 71): the count comes from paging the subscribers endpoint.
  - Member growth comes from welcome_sent_at, the provisioning timestamp —
    the closest thing the portal stores to a join date. Manual/founder members
    have no welcome email, so backfilled days are a documented LOWER BOUND;
    today's row carries the exact count.
  - Runs as an in-app scheduler task (addons.scheduler, 03:15 UTC) rather than
    an agent cron: member-facing plumbing should not depend on the agent VM.
  - Backfill is idempotent; member growth is reconstructible back to
    September and spend/requests back to the gateway's first request.

Charts land later, on the surfaces, once the shape of the series is reviewed.

app/main.py

@@ -607,6 +607,24 @@ def get_db() -> sqlite3.Connection:
         "active INTEGER DEFAULT 1"
         ")"
     )
+    # Daily metrics snapshot, written by /app/code/metrics_snapshot.py from the
+    # scheduler and read by the admin panel + dashboard. Cumulative columns
+    # (members_*, newsletter_subs, matrix_space_members) are counts AS OF that
+    # day; spend_usd/requests are THAT day's traffic. A NULL column means the
+    # source could not be read that day — charts must show a gap, never a zero.
+    conn.execute(
+        "CREATE TABLE IF NOT EXISTS metrics_daily ("
+        "day TEXT PRIMARY KEY, "
+        "members_active INTEGER, "
+        "members_total INTEGER, "
+        "members_new INTEGER, "
+        "newsletter_subs INTEGER, "
+        "matrix_space_members INTEGER, "
+        "spend_usd REAL, "
+        "requests INTEGER, "
+        "recorded_at TEXT"
+        ")"
+    )
     # Member-managed API keys (created via the broker). Each row is one API key
     # under the member's team, tracked so we can list/revoke by name without
     # exposing the chat key. `sk_token` is the full sk- key (revealed once).
@@ -3036,6 +3054,48 @@ async def admin_overview(request: Request):
     }
 
 
+@app.get("/admin/metrics/series")
+@limiter.limit("30/minute")
+async def admin_metrics_series(request: Request, days: int = 90):
+    """Daily metrics series for charts.
+
+    Authenticated by the X-Admin-Token header. Returns oldest-first rows from
+    `metrics_daily` for the last `days` days, for the admin panel and the member
+    dashboard to chart from one source of truth. A NULL column means that source
+    could not be read on that day: render a gap, not a zero.
+
+    Cumulative columns (members_*, newsletter_subs, matrix_space_members) are
+    counts as of that day; spend_usd/requests are that day's traffic only.
+    Backfilled days carry a LOWER BOUND for members_total (members provisioned
+    without a welcome email are absent from the history).
+    """
+    _verify_admin_token(request)
+    limit = max(1, min(int(days or 90), 730))
+    conn = get_db()
+    try:
+        rows = conn.execute(
+            "SELECT day, members_active, members_total, members_new, newsletter_subs, "
+            "matrix_space_members, spend_usd, requests, recorded_at "
+            "FROM metrics_daily ORDER BY day DESC LIMIT ?",
+            (limit,),
+        ).fetchall()
+    finally:
+        conn.close()
+    cols = [
+        "day",
+        "members_active",
+        "members_total",
+        "members_new",
+        "newsletter_subs",
+        "matrix_space_members",
+        "spend_usd",
+        "requests",
+        "recorded_at",
+    ]
+    series = [dict(zip(cols, row)) for row in reversed(rows)]
+    return {"series": series, "count": len(series), "days": limit}
+
+
 @app.get("/admin/model-usage")
 @limiter.limit("30/minute")
 async def admin_model_usage(request: Request, days: int = 0):