admin: internal-only superadmin console (users, payments, DB stats)

New independent `admin` service (admin.main:app) on the same image, reusing
app.db/app.models read-only. Never runs migrations or the scheduler; issues
SELECTs only.

- Password-gated (ADMIN_CONSOLE_PASSWORD) with a 12h signed cookie; closed by
  default when the password is empty.
- Bound to 127.0.0.1:8091 (SSH-tunnel access); off the intranet/NPM network.
- Pages: overview stats, user list + search, per-user history/payment detail,
  DB usage (information_schema size + row estimates).
- Compose: base `admin` service (+prod DB-host override, test mount); Dockerfile
  bakes admin/ into runtime + test stages.
- Tests: tests/test_admin_console.py (auth, queries, page wiring) — 12 passing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

admin/templates/db.html

@@ -0,0 +1,31 @@
+{% extends "base.html" %}
+{% block title %}database{% endblock %}
+{% block body %}
+<h1>Database usage</h1>
+<div class="cards">
+  <div class="card"><div class="n">{{ stats.total_bytes|bytes }}</div><div class="l">Total size</div></div>
+  <div class="card"><div class="n">{{ "{:,}".format(stats.total_rows) }}</div><div class="l">Total rows (est.)</div></div>
+  <div class="card"><div class="n">{{ stats.tables|length }}</div><div class="l">Tables</div></div>
+</div>
+
+<h2>Per table</h2>
+<table>
+  <thead><tr><th>Table</th><th>Rows (est.)</th><th>Data</th><th>Index</th><th>Total</th><th style="width:160px">Share</th></tr></thead>
+  <tbody>
+  {% for t in stats.tables %}
+    <tr>
+      <td>{{ t.name }}</td>
+      <td>{{ "{:,}".format(t.rows) }}</td>
+      <td class="muted">{{ t.data_bytes|bytes }}</td>
+      <td class="muted">{{ t.index_bytes|bytes }}</td>
+      <td>{{ t.total_bytes|bytes }}</td>
+      <td>
+        <div class="bar"><span style="width:{{ t.pct }}%"></span></div>
+        <span class="muted">{{ t.pct }}%</span>
+      </td>
+    </tr>
+  {% endfor %}
+  </tbody>
+</table>
+<p class="muted">Row counts are the storage engine's estimate for InnoDB; sizes are exact on-disk bytes.</p>
+{% endblock %}