Split webui into Flask blueprints and data domain modules

- Split app.py (66 routes) into 3 blueprints: pages (public), api (JSON), admin (@admin_required)
- Split data.py (4,360 LOC) into 7 domain modules: drafts, authors, ratings, gaps, analysis, search, proposals
- Add data/__init__.py re-exporting all public functions for backward compatibility
- Add custom 404/500 error pages matching dark theme
- Add request timing logging via before_request/after_request hooks
- Refactor app.py into create_app() factory pattern
- All 106 tests pass, all 66 routes preserved

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/webui/templates/errors/404.html

@@ -0,0 +1,23 @@
+{% extends "base.html" %}
+
+{% block title %}404 — Not Found{% endblock %}
+
+{% block content %}
+<div class="flex items-center justify-center min-h-[60vh]">
+    <div class="text-center max-w-lg">
+        <h1 class="text-8xl font-bold text-gray-600 mb-4">404</h1>
+        <h2 class="text-2xl font-semibold text-gray-300 mb-4">Page Not Found</h2>
+        <p class="text-gray-400 mb-8">
+            The page you're looking for doesn't exist or has been moved.
+        </p>
+        <div class="flex gap-4 justify-center">
+            <a href="/" class="px-6 py-2 bg-blue-600 hover:bg-blue-700 text-white rounded-lg transition">
+                Back to Overview
+            </a>
+            <a href="/search" class="px-6 py-2 bg-gray-700 hover:bg-gray-600 text-gray-200 rounded-lg transition">
+                Search Drafts
+            </a>
+        </div>
+    </div>
+</div>
+{% endblock %}

src/webui/templates/errors/500.html

@@ -0,0 +1,20 @@
+{% extends "base.html" %}
+
+{% block title %}500 — Server Error{% endblock %}
+
+{% block content %}
+<div class="flex items-center justify-center min-h-[60vh]">
+    <div class="text-center max-w-lg">
+        <h1 class="text-8xl font-bold text-gray-600 mb-4">500</h1>
+        <h2 class="text-2xl font-semibold text-gray-300 mb-4">Internal Server Error</h2>
+        <p class="text-gray-400 mb-8">
+            Something went wrong on our end. Please try again later.
+        </p>
+        <div class="flex gap-4 justify-center">
+            <a href="/" class="px-6 py-2 bg-blue-600 hover:bg-blue-700 text-white rounded-lg transition">
+                Back to Overview
+            </a>
+        </div>
+    </div>
+</div>
+{% endblock %}