Hub: add preferences sync endpoint + notification display on customer page

- POST /api/v1/preferences: accepts {customer_id, email, enabled_events} from controller
- GetRecentNotifications() store method for last N notification log entries
- Customer detail page: new Notifications section (email, events, recent log table)
- joinStrings template function for event list display

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hub/internal/web/templates/customer.html

@@ -155,6 +155,46 @@
             {{end}}
         </section>
 
+        <!-- Notifications -->
+        <section class="card">
+            <h2>Notifications</h2>
+            <div class="info-grid">
+                <div class="info-item">
+                    <span class="label">Email</span>
+                    <span class="value">{{if .NotifPrefs}}{{if .NotifPrefs.Email}}{{.NotifPrefs.Email}}{{else}}Not set{{end}}{{else}}Not configured{{end}}</span>
+                </div>
+                {{if .NotifPrefs}}
+                <div class="info-item">
+                    <span class="label">Events</span>
+                    <span class="value">{{if .NotifPrefs.EnabledEvents}}{{joinStrings .NotifPrefs.EnabledEvents ", "}}{{else}}None{{end}}</span>
+                </div>
+                {{end}}
+            </div>
+            {{if .RecentNotifications}}
+            <h3>Recent (last 10)</h3>
+            <table class="history-table">
+                <thead>
+                    <tr>
+                        <th>Time</th>
+                        <th>Event</th>
+                        <th>Status</th>
+                        <th>Message</th>
+                    </tr>
+                </thead>
+                <tbody>
+                    {{range .RecentNotifications}}
+                    <tr>
+                        <td>{{.CreatedAt.Format "Jan 02 15:04"}}</td>
+                        <td>{{.EventType}}</td>
+                        <td><span class="status-badge status-badge-{{.Status}}">{{.Status}}</span></td>
+                        <td>{{.Message}}</td>
+                    </tr>
+                    {{end}}
+                </tbody>
+            </table>
+            {{end}}
+        </section>
+
         <!-- Report History (last 24h) -->
         {{if .History}}
         <section class="card">