Add Preset Library feature

Presets are saved generation recipes that combine all resource types
(character, outfit, action, style, scene, detailer, look, checkpoint)
with per-field on/off/random toggles. At generation time, entities
marked "random" are picked from the DB and fields marked "random" are
randomly included or excluded.

- Preset model + sync_presets() following existing category pattern
- _resolve_preset_entity() / _resolve_preset_fields() helpers
- Full route set: index, detail, generate, edit, upload, clone, save_json, create (LLM), rescan
- 4 templates: index (gallery), detail (summary + generate), edit (3-way toggle UI), create (LLM form)
- example_01.json reference preset + preset_system.txt LLM prompt
- Presets nav link in layout.html

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

templates/presets/index.html

@@ -0,0 +1,63 @@
+{% extends "layout.html" %}
+
+{% block content %}
+<div class="d-flex justify-content-between align-items-center mb-4">
+    <h2>Preset Library</h2>
+    <div class="d-flex gap-1 align-items-center">
+        <a href="{{ url_for('create_preset') }}" class="btn btn-sm btn-success">Create New Preset</a>
+        <form action="{{ url_for('rescan_presets') }}" method="post" class="d-contents">
+            <button type="submit" class="btn btn-sm btn-outline-primary btn-icon" data-bs-toggle="tooltip" data-bs-placement="bottom" title="Rescan preset files from disk"><img src="{{ url_for('static', filename='icons/refresh.png') }}"></button>
+        </form>
+    </div>
+</div>
+
+{% with messages = get_flashed_messages(with_categories=true) %}
+{% if messages %}
+    {% for category, message in messages %}
+    <div class="alert alert-{{ 'danger' if category == 'error' else 'success' }} alert-dismissible fade show">{{ message }}<button type="button" class="btn-close" data-bs-dismiss="alert"></button></div>
+    {% endfor %}
+{% endif %}
+{% endwith %}
+
+<div class="row row-cols-2 row-cols-sm-3 row-cols-md-4 row-cols-lg-5 row-cols-xl-6 g-3">
+    {% for preset in presets %}
+    <div class="col">
+        <div class="card h-100 character-card" onclick="window.location.href='{{ url_for('preset_detail', slug=preset.slug) }}'">
+            <div class="img-container">
+                {% if preset.image_path %}
+                <img src="{{ url_for('static', filename='uploads/' + preset.image_path) }}" alt="{{ preset.name }}">
+                {% else %}
+                <span class="text-muted">No Image</span>
+                {% endif %}
+            </div>
+            <div class="card-body p-2">
+                <h6 class="card-title text-center mb-1">{{ preset.name }}</h6>
+                <p class="card-text small text-center text-muted mb-0">
+                    {% set parts = [] %}
+                    {% if preset.data.character and preset.data.character.character_id %}
+                        {% set _ = parts.append(preset.data.character.character_id | replace('_', ' ') | title) %}
+                    {% endif %}
+                    {% if preset.data.action and preset.data.action.action_id %}
+                        {% set _ = parts.append('+ action') %}
+                    {% endif %}
+                    {% if preset.data.scene and preset.data.scene.scene_id %}
+                        {% set _ = parts.append('+ scene') %}
+                    {% endif %}
+                    {{ parts | join(' · ') }}
+                </p>
+            </div>
+            <div class="card-footer d-flex justify-content-between align-items-center p-1">
+                <small class="text-muted">preset</small>
+                <div class="d-flex gap-1">
+                    <a href="{{ url_for('edit_preset', slug=preset.slug) }}" class="btn btn-xs btn-outline-secondary" onclick="event.stopPropagation()">Edit</a>
+                </div>
+            </div>
+        </div>
+    </div>
+    {% else %}
+    <div class="col-12">
+        <p class="text-muted">No presets found. <a href="{{ url_for('create_preset') }}">Create your first preset</a> or add JSON files to <code>data/presets/</code> and rescan.</p>
+    </div>
+    {% endfor %}
+</div>
+{% endblock %}