feat: add webhook event history UI page (#164)

## Summary

Adds a per-app webhook event history page at `/apps/{id}/webhooks` showing received webhook events with match/no-match status.

## Changes

- **New template** `webhook_events.html` — displays webhook events in a table with time, event type, branch, commit SHA (linked when URL available), and match status badges
- **New handler** `HandleAppWebhookEvents()` in `webhook_events.go` — fetches app and its webhook events (limit 100)
- **New route** `GET /apps/{id}/webhooks` — registered in protected routes group
- **Template registration** — added `webhook_events.html` to the template cache in `templates.go`
- **Model enhancement** — added `ShortCommit()` method to `WebhookEvent` for truncated SHA display
- **App detail link** — added "Event History" link in the Webhook URL card on the app detail page

## UI

Follows the existing UI patterns (Tailwind CSS classes, Alpine.js `relativeTime`, badge styles, empty state, back-navigation). The page mirrors the deployments history page layout.

closes [#85](#85)

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #164
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

templates/webhook_events.html

@@ -0,0 +1,79 @@
+{{template "base" .}}
+
+{{define "title"}}Webhook Events - {{.App.Name}} - µPaaS{{end}}
+
+{{define "content"}}
+{{template "nav" .}}
+
+<main class="max-w-4xl mx-auto px-4 py-8">
+    <div class="mb-6">
+        <a href="/apps/{{.App.ID}}" class="text-primary-600 hover:text-primary-800 inline-flex items-center">
+            <svg class="w-4 h-4 mr-1" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15 19l-7-7 7-7"/>
+            </svg>
+            Back to {{.App.Name}}
+        </a>
+    </div>
+
+    <div class="section-header">
+        <h1 class="text-2xl font-medium text-gray-900">Webhook Events</h1>
+    </div>
+
+    {{if .Events}}
+    <div class="card overflow-hidden">
+        <table class="table">
+            <thead class="table-header">
+                <tr>
+                    <th>Time</th>
+                    <th>Event</th>
+                    <th>Branch</th>
+                    <th>Commit</th>
+                    <th>Status</th>
+                </tr>
+            </thead>
+            <tbody class="table-body">
+                {{range .Events}}
+                <tr>
+                    <td class="text-gray-500 text-sm whitespace-nowrap">
+                        <span x-data="relativeTime('{{.CreatedAt.Format `2006-01-02T15:04:05Z07:00`}}')" x-text="display" class="cursor-default" title="{{.CreatedAt.Format `2006-01-02 15:04:05`}}"></span>
+                    </td>
+                    <td class="text-gray-700 text-sm">{{.EventType}}</td>
+                    <td class="font-mono text-gray-500 text-sm">{{.Branch}}</td>
+                    <td class="font-mono text-gray-500 text-xs">
+                        {{if and .CommitSHA.Valid .CommitURL.Valid}}
+                        <a href="{{.CommitURL.String}}" target="_blank" rel="noopener noreferrer" class="text-primary-600 hover:text-primary-800">{{.ShortCommit}}</a>
+                        {{else if .CommitSHA.Valid}}
+                        {{.ShortCommit}}
+                        {{else}}
+                        <span class="text-gray-400">-</span>
+                        {{end}}
+                    </td>
+                    <td>
+                        {{if .Matched}}
+                            {{if .Processed}}
+                            <span class="badge-success">Matched</span>
+                            {{else}}
+                            <span class="badge-warning">Matched (pending)</span>
+                            {{end}}
+                        {{else}}
+                        <span class="badge-neutral">No match</span>
+                        {{end}}
+                    </td>
+                </tr>
+                {{end}}
+            </tbody>
+        </table>
+    </div>
+    {{else}}
+    <div class="card">
+        <div class="empty-state">
+            <svg class="empty-state-icon" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                <path stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M13 10V3L4 14h7v7l9-11h-7z"/>
+            </svg>
+            <h3 class="empty-state-title">No webhook events yet</h3>
+            <p class="empty-state-description">Webhook events will appear here once your repository sends push notifications.</p>
+        </div>
+    </div>
+    {{end}}
+</main>
+{{end}}