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/sources_list.html

@@ -0,0 +1,49 @@
+{{template "base" .}}
+
+{{define "title"}}Sources - Webhooker{{end}}
+
+{{define "content"}}
+<div class="max-w-6xl mx-auto px-6 py-8">
+    <div class="flex justify-between items-center mb-6">
+        <h1 class="text-2xl font-medium text-gray-900">Webhooks</h1>
+        <a href="/sources/new" class="btn-primary">
+            <svg class="w-5 h-5 mr-1" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 4v16m8-8H4"/>
+            </svg>
+            New Webhook
+        </a>
+    </div>
+
+    {{if .Webhooks}}
+    <div class="grid gap-4">
+        {{range .Webhooks}}
+        <a href="/source/{{.ID}}" class="card-elevated p-6 block">
+            <div class="flex justify-between items-start">
+                <div>
+                    <h2 class="text-lg font-medium text-gray-900">{{.Name}}</h2>
+                    {{if .Description}}
+                    <p class="text-sm text-gray-500 mt-1">{{.Description}}</p>
+                    {{end}}
+                </div>
+                <span class="badge-info">{{.RetentionDays}}d retention</span>
+            </div>
+            <div class="flex gap-6 mt-4 text-sm text-gray-500">
+                <span>{{.EntrypointCount}} entrypoint{{if ne .EntrypointCount 1}}s{{end}}</span>
+                <span>{{.TargetCount}} target{{if ne .TargetCount 1}}s{{end}}</span>
+                <span>{{.EventCount}} event{{if ne .EventCount 1}}s{{end}}</span>
+            </div>
+        </a>
+        {{end}}
+    </div>
+    {{else}}
+    <div class="card p-12 text-center">
+        <svg class="w-16 h-16 text-gray-300 mx-auto mb-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M13.828 10.172a4 4 0 00-5.656 0l-4 4a4 4 0 105.656 5.656l1.102-1.101m-.758-4.899a4 4 0 005.656 0l4-4a4 4 0 00-5.656-5.656l-1.1 1.1"/>
+        </svg>
+        <h2 class="text-lg font-medium text-gray-900 mb-2">No webhooks yet</h2>
+        <p class="text-gray-500 mb-6">Create your first webhook to start receiving and forwarding events.</p>
+        <a href="/sources/new" class="btn-primary">Create Webhook</a>
+    </div>
+    {{end}}
+</div>
+{{end}}