feat: add encrypted sync user management

Adds full CRUD for sync users (create/list/detail/edit) behind admin
auth and CSRF protection. Passwords are encrypted at rest and never
decrypted into a template context; only emails may be decrypted for
display on the edit form. Blank password fields on edit preserve the
existing encrypted password.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/web/templates/users/detail.html

@@ -0,0 +1,31 @@
+{% extends "base.html" %}
+
+{% block title %}{{ user.name }} - MyWhoosh Garmin Sync{% endblock %}
+
+{% block content %}
+<h1>{{ user.name }}</h1>
+<p><a href="/users/{{ user.id }}/edit">Edit</a> | <a href="/">Back to dashboard</a></p>
+
+<dl>
+    <dt>Enabled</dt>
+    <dd>{{ "Yes" if user.enabled else "No" }}</dd>
+
+    <dt>Health state</dt>
+    <dd>{{ user.health_state.value }}</dd>
+
+    <dt>MyWhoosh state</dt>
+    <dd>{{ user.mywhoosh_state }}</dd>
+
+    <dt>Garmin state</dt>
+    <dd>{{ user.garmin_state }}</dd>
+
+    <dt>Action reason</dt>
+    <dd>{{ user.action_reason or "-" }}</dd>
+
+    <dt>Created at</dt>
+    <dd>{{ user.created_at }}</dd>
+
+    <dt>Updated at</dt>
+    <dd>{{ user.updated_at }}</dd>
+</dl>
+{% endblock %}