feat: add admin panel with session auth, user management, and billing dashboard

Implement a full-featured admin backend at `/manage/` with login authentication using
signed cookies and constant-time credential verification. The panel includes a dashboard
showing total/active users, storage usage, monthly revenue, and pending invoices, plus
user management, payment overview, and pricing configuration pages. New environment
variables `ADMIN_USERNAME`, `ADMIN_PASSWORD`, `ADMIN_SESSION_SECRET`, and
`ADMIN_SESSION_EXPIRE_HOURS` control access. The admin router is registered in main.py
and all new templates, auth module, and router files are added.

mywebdav/templates/admin/dashboard.html

@@ -0,0 +1,76 @@
+{% extends "admin/base.html" %}
+
+{% block title %}Dashboard{% endblock %}
+
+{% block content %}
+<div class="page-header">
+    <h1 class="page-title">Dashboard</h1>
+    <p class="page-subtitle">Overview of your MyWebdav instance</p>
+</div>
+
+<div class="stats-grid">
+    <div class="stat-card">
+        <div class="stat-label">Total Users</div>
+        <div class="stat-value">{{ stats.total_users }}</div>
+    </div>
+    <div class="stat-card">
+        <div class="stat-label">Active Users</div>
+        <div class="stat-value success">{{ stats.active_users }}</div>
+    </div>
+    <div class="stat-card">
+        <div class="stat-label">Total Storage Used</div>
+        <div class="stat-value">{{ stats.total_storage | format_bytes }}</div>
+    </div>
+    <div class="stat-card">
+        <div class="stat-label">Monthly Revenue</div>
+        <div class="stat-value success">{{ stats.monthly_revenue | format_currency }}</div>
+    </div>
+    <div class="stat-card">
+        <div class="stat-label">Pending Invoices</div>
+        <div class="stat-value {% if stats.pending_invoices > 0 %}warning{% endif %}">{{ stats.pending_invoices }}</div>
+    </div>
+    <div class="stat-card">
+        <div class="stat-label">Inactive Users</div>
+        <div class="stat-value {% if stats.inactive_users > 0 %}danger{% endif %}">{{ stats.inactive_users }}</div>
+    </div>
+</div>
+
+<div class="detail-grid">
+    <div class="card">
+        <div class="card-header">
+            <h2 class="card-title">Quick Actions</h2>
+        </div>
+        <div style="display: flex; flex-wrap: wrap; gap: 12px;">
+            <a href="/manage/users" class="btn btn-primary">Manage Users</a>
+            <a href="/manage/payments" class="btn btn-outline">View Payments</a>
+            <a href="/manage/settings" class="btn btn-outline">Settings</a>
+        </div>
+    </div>
+
+    <div class="card">
+        <div class="card-header">
+            <h2 class="card-title">Recent Activity</h2>
+        </div>
+        {% if recent_activities %}
+        <div class="activity-list">
+            {% for activity in recent_activities %}
+            <div class="activity-item">
+                <div class="activity-icon">{{ activity.user[0] | upper }}</div>
+                <div class="activity-content">
+                    <div class="activity-text">
+                        <strong>{{ activity.user }}</strong> {{ activity.action }}
+                    </div>
+                    <div class="activity-time">{{ activity.timestamp.strftime('%Y-%m-%d %H:%M') }}</div>
+                </div>
+            </div>
+            {% endfor %}
+        </div>
+        {% else %}
+        <div class="empty-state">
+            <div class="empty-state-icon">📋</div>
+            <div class="empty-state-text">No recent activity</div>
+        </div>
+        {% endif %}
+    </div>
+</div>
+{% endblock %}