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>

routes/__init__.py

@@ -17,6 +17,7 @@ def register_routes(app):
     from routes import gallery
     from routes import strengths
     from routes import transfer
+    from routes import api
 
     queue_api.register_routes(app)
     settings.register_routes(app)
@@ -35,3 +36,4 @@ def register_routes(app):
     gallery.register_routes(app)
     strengths.register_routes(app)
     transfer.register_routes(app)
+    api.register_routes(app)