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/login.html

@@ -0,0 +1,14 @@
+{% extends "base.html" %}
+{% set show_nav = false %}
+{% block title %}login{% endblock %}
+{% block body %}
+<div class="login">
+  <h1>Superadmin</h1>
+  {% if error %}<div class="err">{{ error }}</div>{% endif %}
+  <form method="post" action="/login">
+    <p><input type="password" name="password" placeholder="Password"
+              autofocus autocomplete="current-password" style="width:100%"></p>
+    <button type="submit">Sign in</button>
+  </form>
+</div>
+{% endblock %}