feat: implement core webhook engine, delivery system, and management UI (Phase 2)

- Webhook reception handler: look up entrypoint by UUID, verify active,
  capture full HTTP request (method, headers, body, content-type), create
  Event record, queue Delivery records for each active Target, return 200 OK.
  Handles edge cases: unknown UUID → 404, inactive → 410, oversized → 413.

- Delivery engine (internal/delivery): fx-managed background goroutine that
  polls for pending/retrying deliveries and dispatches to target type handlers.
  Graceful shutdown via context cancellation.

- Target type implementations:
  - HTTP: fire-and-forget POST with original headers forwarding
  - Retry: exponential backoff (1s, 2s, 4s...) up to max_retries
  - Database: immediate success (event already stored)
  - Log: slog output with event details

- Webhook management pages with Tailwind CSS + Alpine.js:
  - List (/sources): webhooks with entrypoint/target/event counts
  - Create (/sources/new): form with auto-created default entrypoint
  - Detail (/source/{id}): config, entrypoints, targets, recent events
  - Edit (/source/{id}/edit): name, description, retention_days
  - Delete (/source/{id}/delete): soft-delete with child records
  - Add Entrypoint (/source/{id}/entrypoints): inline form
  - Add Target (/source/{id}/targets): type-aware form
  - Event Log (/source/{id}/logs): paginated with delivery status

- Updated README: marked completed items, updated naming conventions
  table, added delivery engine to package layout and DI docs, updated
  column names to reflect entity rename.

- Rebuilt Tailwind CSS for new template classes.

Part of: #15

templates/source_logs.html

@@ -0,0 +1,61 @@
+{{template "base" .}}
+
+{{define "title"}}Event Log - {{.Webhook.Name}} - Webhooker{{end}}
+
+{{define "content"}}
+<div class="max-w-6xl mx-auto px-6 py-8">
+    <div class="mb-6">
+        <a href="/source/{{.Webhook.ID}}" class="text-sm text-primary-600 hover:text-primary-700">&larr; Back to {{.Webhook.Name}}</a>
+        <div class="flex justify-between items-center mt-2">
+            <h1 class="text-2xl font-medium text-gray-900">Event Log</h1>
+            <span class="text-sm text-gray-500">{{.TotalEvents}} total event{{if ne .TotalEvents 1}}s{{end}}</span>
+        </div>
+    </div>
+
+    <div class="card">
+        <div class="divide-y divide-gray-100">
+            {{range .Events}}
+            <div class="p-4" x-data="{ open: false }">
+                <div class="flex items-center justify-between cursor-pointer" @click="open = !open">
+                    <div class="flex items-center gap-3">
+                        <span class="badge-info">{{.Method}}</span>
+                        <span class="text-sm font-mono text-gray-700">{{.ID}}</span>
+                        <span class="text-sm text-gray-500">{{.ContentType}}</span>
+                    </div>
+                    <div class="flex items-center gap-4">
+                        {{range .Deliveries}}
+                        <span class="text-xs {{if eq .Status "delivered"}}text-green-600{{else if eq .Status "failed"}}text-red-600{{else if eq .Status "retrying"}}text-yellow-600{{else}}text-gray-400{{end}}">
+                            {{.Target.Name}}: {{.Status}}
+                        </span>
+                        {{end}}
+                        <span class="text-xs text-gray-400">{{.CreatedAt.Format "2006-01-02 15:04:05"}}</span>
+                        <svg class="w-4 h-4 text-gray-400 transition-transform" :class="{ 'rotate-180': open }" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M19 9l-7 7-7-7"/>
+                        </svg>
+                    </div>
+                </div>
+
+                <div x-show="open" x-cloak class="mt-3 p-3 bg-gray-50 rounded-md">
+                    <pre class="text-xs text-gray-700 overflow-x-auto whitespace-pre-wrap break-all">{{.Body}}</pre>
+                </div>
+            </div>
+            {{else}}
+            <div class="p-12 text-center text-sm text-gray-500">No events recorded yet.</div>
+            {{end}}
+        </div>
+    </div>
+
+    <!-- Pagination -->
+    {{if or .HasPrev .HasNext}}
+    <div class="flex justify-center gap-2 mt-6">
+        {{if .HasPrev}}
+        <a href="/source/{{.Webhook.ID}}/logs?page={{.PrevPage}}" class="btn-secondary text-sm">&larr; Previous</a>
+        {{end}}
+        <span class="inline-flex items-center px-4 py-2 text-sm text-gray-500">Page {{.Page}} of {{.TotalPages}}</span>
+        {{if .HasNext}}
+        <a href="/source/{{.Webhook.ID}}/logs?page={{.NextPage}}" class="btn-secondary text-sm">Next &rarr;</a>
+        {{end}}
+    </div>
+    {{end}}
+</div>
+{{end}}