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/api.py

@@ -0,0 +1,105 @@
+import logging
+from functools import wraps
+from flask import request, jsonify
+from models import Preset, Settings
+from services.generation import generate_from_preset
+from services.job_queue import _job_queue_lock, _job_history
+
+logger = logging.getLogger('gaze')
+
+
+def register_routes(app):
+
+    def require_api_key(f):
+        @wraps(f)
+        def decorated(*args, **kwargs):
+            api_key = request.headers.get('X-API-Key') or request.args.get('api_key')
+            if not api_key:
+                return jsonify({'error': 'Missing API key. Provide X-API-Key header or api_key query parameter.'}), 401
+            settings = Settings.query.first()
+            if not settings or not settings.api_key or api_key != settings.api_key:
+                return jsonify({'error': 'Invalid API key.'}), 403
+            return f(*args, **kwargs)
+        return decorated
+
+    @app.route('/api/v1/presets')
+    @require_api_key
+    def api_v1_presets():
+        presets = Preset.query.order_by(Preset.name).all()
+        return jsonify({
+            'presets': [
+                {
+                    'preset_id': p.preset_id,
+                    'slug': p.slug,
+                    'name': p.name,
+                    'has_cover': bool(p.image_path),
+                }
+                for p in presets
+            ]
+        })
+
+    @app.route('/api/v1/generate/<path:preset_slug>', methods=['POST'])
+    @require_api_key
+    def api_v1_generate(preset_slug):
+        preset = Preset.query.filter_by(slug=preset_slug).first()
+        if not preset:
+            return jsonify({'error': f'Preset not found: {preset_slug}'}), 404
+
+        body = request.get_json(silent=True) or {}
+
+        count = body.get('count', 1)
+        try:
+            count = int(count)
+        except (TypeError, ValueError):
+            return jsonify({'error': 'count must be an integer'}), 400
+        if count < 1 or count > 20:
+            return jsonify({'error': 'count must be between 1 and 20'}), 400
+
+        seed = body.get('seed')
+        if seed is not None:
+            try:
+                seed = int(seed)
+            except (TypeError, ValueError):
+                return jsonify({'error': 'seed must be an integer'}), 400
+
+        width = body.get('width')
+        height = body.get('height')
+        if (width is None) != (height is None):
+            return jsonify({'error': 'width and height must both be provided or both omitted'}), 400
+
+        overrides = {
+            'action': 'preview',
+            'checkpoint': body.get('checkpoint', ''),
+            'extra_positive': body.get('extra_positive', ''),
+            'extra_negative': body.get('extra_negative', ''),
+            'seed': seed,
+            'width': int(width) if width else None,
+            'height': int(height) if height else None,
+        }
+
+        try:
+            jobs = []
+            for _ in range(count):
+                job = generate_from_preset(preset, overrides)
+                jobs.append({'job_id': job['id'], 'status': 'queued'})
+            return jsonify({'jobs': jobs}), 202
+        except Exception as e:
+            logger.exception("API generation error (preset %s): %s", preset_slug, e)
+            return jsonify({'error': str(e)}), 500
+
+    @app.route('/api/v1/job/<job_id>')
+    @require_api_key
+    def api_v1_job_status(job_id):
+        with _job_queue_lock:
+            job = _job_history.get(job_id)
+            if not job:
+                return jsonify({'error': 'Job not found'}), 404
+            resp = {
+                'id': job['id'],
+                'label': job['label'],
+                'status': job['status'],
+                'error': job['error'],
+            }
+            if job.get('result'):
+                resp['result'] = job['result']
+            return jsonify(resp)