Record origin changes and show them on the prefix page (closes #161)

The routing table records each prefix and origin AS pair's origin changes:
when it gains its first peer and when it loses its last. Each row holds the
prefix, the origin AS, announced or withdrawn, the peer count and the time.
The prefix handler writes them to a new origin_changes table, and the database
maintainer prunes each prefix to its newest 500 rows and 72 hours. The prefix
page lists them newest first, at most 5,000.

Model: deepseek-v4-pro

internal/templates/prefix_detail.html

@@ -297,6 +297,41 @@
                 {{end}}
             </div>
         </div>
+{{end}}
+
+        {{if .OriginChanges}}
+        <div class="origins-section">
+            <h3>Origin Changes</h3>
+            {{if .MoreOriginChanges}}
+            <p>There are more than 5,000 changes. Showing the newest 5,000.</p>
+            {{end}}
+            <table class="route-table">
+                <thead>
+                    <tr>
+                        <th>Time (UTC)</th>
+                        <th>Change</th>
+                        <th>Origin AS</th>
+                        <th>Peers</th>
+                    </tr>
+                </thead>
+                <tbody>
+                    {{range .OriginChanges}}
+                    <tr>
+                        <td>{{.Time.Format "2006-01-02 15:04:05"}}</td>
+                        <td>{{if .Announced}}Announced{{else}}Withdrawn{{end}}</td>
+                        <td>
+                            {{if .OriginASN}}
+                            <a href="/as/{{.OriginASN}}" class="as-link">AS{{.OriginASN}}</a>
+                            {{else}}
+                            No known origin
+                            {{end}}
+                        </td>
+                        <td>{{.PeerCount}}</td>
+                    </tr>
+                    {{end}}
+                </tbody>
+            </table>
+        </div>
         {{end}}
 
         {{if .Routes}}