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_new.html

@@ -0,0 +1,41 @@
+{{template "base" .}}
+
+{{define "title"}}New Webhook - Webhooker{{end}}
+
+{{define "content"}}
+<div class="max-w-2xl mx-auto px-6 py-8">
+    <div class="mb-6">
+        <a href="/sources" class="text-sm text-primary-600 hover:text-primary-700">&larr; Back to webhooks</a>
+        <h1 class="text-2xl font-medium text-gray-900 mt-2">Create Webhook</h1>
+    </div>
+
+    <div class="card p-6">
+        {{if .Error}}
+        <div class="alert-error">{{.Error}}</div>
+        {{end}}
+
+        <form method="POST" action="/sources/new" class="space-y-6">
+            <div class="form-group">
+                <label for="name" class="label">Name</label>
+                <input type="text" id="name" name="name" required autofocus placeholder="My Webhook" class="input">
+            </div>
+
+            <div class="form-group">
+                <label for="description" class="label">Description</label>
+                <textarea id="description" name="description" rows="3" placeholder="Optional description" class="input"></textarea>
+            </div>
+
+            <div class="form-group">
+                <label for="retention_days" class="label">Retention (days)</label>
+                <input type="number" id="retention_days" name="retention_days" value="30" min="1" max="365" class="input">
+                <p class="text-xs text-gray-500 mt-1">How long to keep event data.</p>
+            </div>
+
+            <div class="flex gap-3">
+                <button type="submit" class="btn-primary">Create Webhook</button>
+                <a href="/sources" class="btn-secondary">Cancel</a>
+            </div>
+        </form>
+    </div>
+</div>
+{{end}}