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>

models.py

@@ -291,6 +291,8 @@ class Settings(db.Model):
     checkpoint_dirs     = db.Column(db.String(1000), default='/ImageModels/Stable-diffusion/Illustrious,/ImageModels/Stable-diffusion/Noob')
     # Default checkpoint path (persisted across server restarts)
     default_checkpoint  = db.Column(db.String(500), nullable=True)
+    # API key for REST API authentication
+    api_key = db.Column(db.String(255), nullable=True)
 
     def __repr__(self):
         return '<Settings>'