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/scenes/index.html

@@ -29,9 +29,15 @@
                 <img id="img-{{ scene.slug }}" src="{{ url_for('static', filename='uploads/' + scene.image_path) }}" alt="{{ scene.name }}">
                 <span id="no-img-{{ scene.slug }}" class="text-muted d-none">No Image</span>
                 {% else %}
+                {% set fallback = random_gen_image('scenes', scene.slug) %}
+                {% if fallback %}
+                <img id="img-{{ scene.slug }}" src="{{ url_for('static', filename='uploads/' + fallback) }}" alt="{{ scene.name }}" class="fallback-cover">
+                <span id="no-img-{{ scene.slug }}" class="text-muted d-none">No Image</span>
+                {% else %}
                 <img id="img-{{ scene.slug }}" src="" alt="{{ scene.name }}" class="d-none">
                 <span id="no-img-{{ scene.slug }}" class="text-muted">No Image</span>
                 {% endif %}
+                {% endif %}
             </div>
             <div class="card-body">
                 <h5 class="card-title text-center">{{ scene.name }}</h5>