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>

app.py

@@ -106,6 +106,7 @@ if __name__ == '__main__':
             ('lora_dir_detailers',  "VARCHAR(500) DEFAULT '/ImageModels/lora/Illustrious/Detailers'"),
             ('checkpoint_dirs',     "VARCHAR(1000) DEFAULT '/ImageModels/Stable-diffusion/Illustrious,/ImageModels/Stable-diffusion/Noob'"),
             ('default_checkpoint',  "VARCHAR(500)"),
+            ('api_key',             "VARCHAR(255)"),
         ]
         for col_name, col_type in columns_to_add:
             try: