Add REST API for preset-based generation and fallback cover images

REST API (routes/api.py): Three endpoints behind API key auth for
programmatic image generation via presets — list presets, queue
generation with optional overrides, and poll job status.

Shared generation logic extracted from routes/presets.py into
services/generation.py so both web UI and API use the same code path.

Fallback covers: library index pages now show a random generated image
at reduced opacity when no cover is assigned, instead of "No Image".

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

templates/presets/index.html

@@ -27,8 +27,13 @@
                 {% if preset.image_path %}
                 <img src="{{ url_for('static', filename='uploads/' + preset.image_path) }}" alt="{{ preset.name }}">
                 {% else %}
+                {% set fallback = random_gen_image('presets', preset.slug) %}
+                {% if fallback %}
+                <img src="{{ url_for('static', filename='uploads/' + fallback) }}" alt="{{ preset.name }}" class="fallback-cover">
+                {% else %}
                 <span class="text-muted">No Image</span>
                 {% endif %}
+                {% endif %}
             </div>
             <div class="card-body p-2">
                 <h6 class="card-title text-center mb-1">{{ preset.name }}</h6>